original W6's and W3's have a standard convex dust cap. A W3 v2 has a concave dustcap.

the W6v1 has a flat magnet on the back with a large, reflective JL sticker - the W3 v1 has a bumped back pole piece.

check JL's site for specs on all JL subs - including discontinued models.

unless you run into something really odd like a W4 or W5 that should cover you. When in doubt, measure the coils - the W6 will have dual 6 ohm coils