How to test quantum mechanics? In this paper, Testing quantum mechanics, S. Weinberg states that

The problem is to know what to test. Usually we can get guidance on
  how to test a theory like general relativity or the standard model of
  elementary particle interac- tions, by first inventing some
  generalized theory as a foil, such as general relativity with extra
  massless scalars, or the standard model with extra gauge bosons. By a
  “generalized” theory, I mean one that differs from the theory we want
  to test, but reduces to it when some parameters become very small. (In
  this sense, the local hidden variable theories do not qualify as
  generalizations of quantum mechanics.) We can set upper bounds on
  these parameters by doing experiments to look for new effects that
  could arise in the generalized theory, and in this way we get a sense
  of how accurate is the theory we want to test.

However, I cannot understand what does he mean by this paragraph? For example, why do we need a "generalized theory" to test the actual theory that we want to test?, and if we have one, how do we use it to test the actual theory? Is there any other way to test it [the actual theory]? 
 A: The paper sounds interesting. Is it available somewhere without a paywall?
He's talking about testing a theory that has already been tested many times in a whole bunch of different contexts and has passed all those tests. Therefore the theory can't just be wrong, like Freudian psychology or Marxism. The only possibility is that it's a special case of some broader theory. This is called the correspondence principle: new theories have to be backward-compatible with old ones. Because of the correspondence principle, my freshman students are not going to disprove quantum mechanics by measuring the spectrum of hydrogen. You have to have some framework to guide you in constructing tests so that you can quantify which conditions are the ones that have not yet been tested, and or what upper limit has already been placed on violations.
It can also be a waste of time to try tests if it's logically impossible for the test to falsify the theory. For instance, suppose I want to test whether probability is conserved, as predicted by standard quantum mechanics. How the heck would I test that? I do the experiment, and something happens. If the total probability of the outcomes was 0.9, then what would that mean? Would the whole universe cease to exist for me with probability 0.1? Or what if the theory says that a certain probability is negative -- how do I test that theory?
Quantum mechanics turns out to be extremely fragile in this sense: it's very difficult to get any viable and different theory by making small perturbations to quantum mechanics. A good paper on this is Aaronson, "Is Quantum Mechanics An Island In Theoryspace?," http://arxiv.org/abs/quant-ph/0401062
