Two bosons having the same state --- how do you know there are two? So, suppose that photons have the same quantum state. How do we know that there are 'two' photons having the same state, rather than just one? 
Is there a technical way to guarantee that there are two photons instead of one, or is it by stipulation? 
Thanks. 
 A: 
How do we know that there are 'two' photons having the same state, rather than just one?

Take the expectation value of the number operator on that state and look at the result. If the result is two then there are two photons. If it is one then there is one.
A: 
Is there a technical way to guarantee that there are two photons instead of one, or is it by stipulation? 

This is very general. If one is talking of the ambient cosmic background radiation photons , a type of photon "gas" of very low frequency, no technical way can be found since the energies are too low to measure individual photons. 
Actually it is only in the creation of photons that they can be counted and verified later in detection. 
Individual photons from atomic emissions can be detected and their state verified  using polarizers and spectrometers. In optical frequencies their position in a spectrum can be detected as a point on a screen.

individual photons will be dots filling up the lines.
An experiment could be devised where within the $Δ(t)$ of the Heisenberg uncertainty two photons with the same spin projection would be detected, though what this would be useful for is debatable.
If you are talking of the theoretical mathematical model representing the photons, the word "know" is irrelevant, one constructs  the model so that this is true.
