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