(How) Can one determine if an exoplanet is synchronously tidally locked or not? Mercury is asynchronously tidally locked so that all of its surface regularly sees the Sun. But an exoplanet which is synchronously locked to its star, could we tell that it is? It would have a hot side and a cold side. This should affect transit and occultation on some wavelengths, but does the symmetry of the movement degenerate any possibility to disentangle tidal locking from that?

Mercury seems to cool down during night time, and gas giants maybe distribute their temperature atmospherically. So their tidal locking might be hard to discern. But there are terrestrial exoplanets with much shorter orbital period which should have a cool side and a hot side permanently. That's my speculation. My basic curiosity is:

Is Mercury a freak or a standard in terms of tidally locked planets being asynchronously so? And by observational support, not just models.
No. Not with the limitations of our current technology. 99% of anything you've heard describing the state, makeup, nature of or appearance, of any exoplanet, is wishful thinking and conjecture. Our current technology is still rather limited. Today, we are able to come up with reasonable guesstimates of the sizes of exoplanets and their distances from their host stars, all with large degrees of uncertainty. That's it - size and distance - nothing else - at least for today (2017).

When we look at a star and see that its light has regular periodic dips, we measure the length of, the strength of, and the period of those dips (that is all we can see). Then we do a lot of math and guess at the size (mass) of the exoplanet and its distance from its host star. Even the size (both mass and diameter) of these planet-hosting stars are mere guesses. Only 16 of all the visible stars have ever been directly imaged (resolved to a disc).

However, people do love to dream. Some scientists LOVE to embellish. Some scientists like to walk around like peacocks with their plumage spread in display, hoping folks won't see their behind behind their tail feathers.
(I got to tell you - I really enjoyed thinking up that last sentence.)