As far as I know, Popeye is very noticible-one or both of the fish's eyes bulge out of its socket and may even become cloudy(like an opaque film over the entire eye.) I was told it was not contagious because it is a result from a bacterial infection or poor water conditions.
Speaking of which, I have a 5 month old molly that has had "popeye/cloudy eye" on one of her eyes fro a couple of weeks.I put her in a hospital tank and treated with maracyn, but it hasnt really helped. I left it for a little to see if it would clear up on its own but it hasent. Other then the weird eye the fish is healthy and active. The water parems are : NitrAte: 0 Nitrite: 0 Ammonia:0 and their is some salt in the water.
Also, before the disease she was very clearly a female, but now i noticed she appears to be becoming a male but without the huge dorsal fin typical of gold dust "sailfin" mollies that her brother has. ?