If they're adults you may be able to tell by their tails - looked at from underneath, a male's tail will bulge more below the vent and won't taper off as quick as a female's does.
Not so easy if they're babies - you may still be able to pop them but if you're not sure how get someone experienced to do it. I wouldn't think about probing unless they're 18 months+.
I think some species may be more difficult to breed than others - what type are yours?
I had baby red-sideds from a pair that were just kept in the same viv. She had 20+, and some species can have more, so if you decide to breed them the babies would need feeding 3 times a week (initially on fish or worms) so be prepared for a hectic (and messy!) few weeks and have plenty of tubs ready in advance just in case you get loads.