* It was consistent with most observations and experiments done thus far.

* It offered an explanation for known issues which were being grappled with at the time, i.e. the observation of speed of light in moving reference frames.

* It built on existing work and showed Einstein's familiarity with the state of physics at that point.

* It was possible to understand how the theory could be tested, even if the technical skills of the time were inadequate.

Compare this to most crackpot theories we see today: they may be inconsistent with previous observations. They claim to debunk "establishment" theories rather than attempting to grapple with new problems. And they are often constructed so they cannot be falsified, even postulating improved experimental techniques.

So if you see a new theory which claims to explain something like high-Tc superconductivity but requires advances in technology to be testable, it is probably a good-faith effort. But if you see a theory which claims to revolutionize physics by means of an unobservable energy field with no plausible experiments, you're probably looking at a crackpot. Bonus points if the author says "Einstein is WRONG!" in a press release.