Can we detect whether food has previously been heated in a microwave oven? An acquaintance told me that she refuses to eat microwaved food because she read that the microwave changes the molecules of the food somehow. Her statement is vague and her sources are dubious but I am curious.
I know that microwaves tend to heat food unevenly, causing changes at the macro scale to the structure of the food, but I am more interested in changes at the micro scale which might effect the food's nutritional properties.
Are there any known, detectable changes that occur when food is heated in a microwave oven? Given a sample of cold food, is there any way to determine whether it has been microwaved at some point in the past?
 A: Your friend is, very, very theoretically, right, but the risks on both theoretical grounds and also epidemiological grounds - i.e. microwave ovens have been used by many people for a long time without obvious illnesses showing themselves - are extremely small.
There are two ways wherein microwave cooking might "change the molecules": the first 


*

*They might break and reconfigure bonds within organic molecules. However, whilst this theoretically happens, it happens unbelievable seldom if practically at all. Bond energies and bond dissociation energies are of the order of electron volts or tens thereof. So they are a few or a few tens of optical photons' worth of energy: bond reconfiguration is thus driven by photons with frequency of the order 1000THz. Microwave oven photons, on the other hand, at 1 to 2 gigahertz, are six orders of magnitude less energetic. However, from quantum mechanics, there is a nonzero probability that bond breaking by microwave photons will happen, but it will be fantastically low. This is the idea of quantum tunnelling: if and event, through energy considerations, is forbidden classically, it still happens, albeit seldom. Cold hydrogen fusion happens, for example, when you pull sticky-tape off something, but the events are fantastically seldom.

*Microwaves denature proteins through their pure heating effect, i.e. change their three dimensional shape without changing the chemical bonds within them. An analogy is supercoiling and curliness in a telephone receiver cable. The basic cable can stay intact, but different amounts of winding can get it "stuck" in configurations of different 3D shape (like the kind where it's supercoiled so much the knots wrap themselves around your hand when you're trying to talk on the telephone and your interlocutor, if unlucky, thinks they're getting sworn at). However, this denaturing is exactly the same effect as wrought by any other kind of heating. Protein denaturing is essentially the difference between cooked food and raw, whatever the heat source used for the cooking was.
So yes, the molecules do change, but in ways that are pretty much the same as changes wrought by any kind of heating, or even folding (as with an egg white - the whitening of whipped egg is owing to mehcanically wrought denaturing).
This article here is a more learned exposition on some of my ideas above.
Edit After Interesting Comment:
User Davidmh made the following comment on Volker's Answer:

Recipe: potatoes sliced in the microwave. Some of them, the ones in contact with the container can get very toasted, as if you grilled them.

This raises an interesting point. Although I believe the potato toasting is still a pure heating effect, there may indeed be an effect at work here that's peculiar to microwave cooking. The food in the microwave is interacting with the electomagnetic radiation, and so there must be a reaction - or scattered - electromagnetic field so that the food changes the field distribution within the resonant cavity. What you're seeing here is probably a combination of all four of the following:


*

*Microwaves are excluded more from deep within a big mass of food than they are from the edges: the changed field configuration means the edges get more heat. This is a bit like the electromagnetic skin effect;

*Near an edge, the food is less well "heat sink-ed". Heat can diffuse off in all directions away from a locally hot region within the body, so the heating tends to be made even within the body of a mass of food. At the edges, there are fewer diffusive paths for the heat to get away from locally hot regions;

*Water boils off in a microwave oven. It cannot get away from the inside of a mass of food, and so it tends to set up water-steam equilibriums inside the food body and thus tend to keep the temperature nearer to 100C. But water can boil off the surface of a food body. So the edges tend to dry out swiftly and, lacking the liquid water-steam equilibrium that tends to constrain the temperature, can rise to a much higher temperature.

*If the container is at all conductive, it will absorb the microwaves and become a local hot spot. I have found some earthenware dishes and pots do tend to get extremely hot on their own in a microwave oven. One of the factors in declaring something "microwave safe" is whether it absorbs in this way: not only does it heat the food unevenly if it does, it can destroy itself.


You could test how much 4. is a factor with a particular pot by putting it into the microwave with nothing in it and seeing whether it heats. BTW make sure you switch the microwave on for the test: I was trying to debug a test setup a few days ago and took two hours to twig that I hadn't switched the power on to a key piece of kit!
A: Maybe it helps to approach that problem in this - somewhat "backward" way:
Putting food in a microwave oven basically just heats it - in a conceptually ideal way, (if it would be uneven in practise.)
But other ways to cook are way more prone to leave artefacts in the food;
like brown, caramelized surface parts, or bigger parts being softer/more cooked on the outer regions than in the centre.
So, assuming we can detect indicators of all other ways to cook,
and we can detect the food was heated at all - eg: it's cooked/done:
If there are no traces of other ways to cook, we can assume that it was 
cooked by microwave.
(Ok, I realize it's a somehow theoretical approach - hey, I have not that much
practical experience with cooking without microwave...)
From the practical perspective, I think you can not detect just heating,
as it does not leave much traces - but you can certainly detect "too strong" heating, like boiling a soup, leaving a dried border in a characteristic way -
(as you write yourself).
A: What happens is called photoelectric effect, the microwave causes electrons in the food (specifically in the water molecules) to go away: this process leads to energy i.e heat.
That's why it is not a good idea to put a metal inside the microwave.
In fact Microwave is healthier because it doesn't vaporize the water molecules. Some nutrients do actually get destroyed but that's due to heat and not microwave.
A: I've no idea if this is possible, I'm just proposing a mechanism:
Microwaves heat food by causing polar molecules (eg water) to vibrate - I'm guessing you could in theory detect the equivalent of magnetisation in the foodstuff, where clusters of molecules have been left aligned by the microwaves?  I doubt the effect would last long in such a noisy (hot!) environment, but it may be possible.
