How can one know if his or her question is non-mainstream? Just by asking it? Or is it supposed that the one who asks knows that for him/herself?
This is any many ways a "we'll know it when we see it" thing, but there are some reliable indicators that you will be able to defend the question from accusations of non-mainstream-ness.
Material that appears in major textbooks. This is golden.
Material that has be treated by more than one research group in major journals. Also golden. (If it is new you might need to give references, and it is going to be hit or miss if anyone here knows anything about it).
Applications of established science to new systems to which they clearly apply. The literature isn't (can't be) completely comprehensive and some extensions are just obvious, but if you get creative people might question you.
Published speculation by individual reputable physicist (or small groups). Here you are getting into the cutting edge, and opinions might vary.
Almost anything else is going to run into questions. And yes, this means the Physics is not a place to develop new ideas. Take it to chat or elsewhere.