[alkonaut]

Most times an experiment isn't possible/practical to perform, but the experiment could at least be described. The Higgs particle "If we build an accelerator that accelerates particles to energies pf X electron volts, we should see traces of it" must have sounded almost like a thought experiment when the Higgs boson was theorized. Same thing for e.g. the General Theory of Relativity and the Mercury passage that was one of the first validations of it.

Clearly if you propose a theory that requires an experiment so advanced (or circumstances so rare) we can't hope to do it in 20 years, it's still a valid scientific theory. What about 50-100 years? What if it requires technology so advanced it's nearly unthinkable that we will ever attain it? This is when it becomes a philiosophical gray area. It's not a clear cut case what is verifiable and what isn't, since the theory is valid before our ability to verify it.