Syncing with focus plane shutter and with central shutter is quite a different thing. For the latter you need to:
- not trigger to early, while the shutter is still closed or opening (the case when you switch the bloody M/X lever to M)
- not induce to much delay, so as flash would be triggered when the shutter is already closing (or closed)
If you triggered while the shutter is opening or closing, you would get underexposed image (and perhaps deeper DOF).
If your flash is fast and your remote trigger doesn't delay, you usually can sync to the shortest time provided by the lens/shutter. That's by the way what central shutters are for.
With a focus plane shutter (most of bodies with TTL-metering) you never (bar using HSS) can go below the X-sync speed, the shortest time when the whole frame is open at a time, otherwise you'd get unexposed strip on the edge of the frame.
It's quite easy to test: in a dark room (not necessarily in total darkness) point the flash and the lens to a wall, and just watch from the other side of the lens how the aperture looks like while flashing: if it's round, it's OK, if it's like a star (or a starfish) it's bad; if you can't see anything, it's even worse.
Don't point the flash directly to the lens. Reduce the power. You may want to repeat the test for several times, and if you flash directly to the lens you may not be able to see anything anymore