Cards will have little cutout circles on the caps, while the omnis won't have any.
This only works if he has the CA-11s... and based on the question he doesn't. The CA-14s have permanently mounted windscreens so you cannot see the holes... which is why Chris Church adds the red and black shrinkfilm.
CA-14 CardioidsMarked with black shrink film at jack.
CA-14 OmnisMarked with red shrink film at jack.