Some Discus are easy to spot a male (ie BD), some are harder. The only sure fire way I know is to take the pair, put them in a tank and if you get babies you have a pair. You have a 50/50 chance of getting it wright. I mean the more experienced members have the eye for spotting a potential pair but other than I don't know. But that's the fun with this hobby for ppl like me, I like cursing up a storm when I find out I either have 2 males/females in a tank hahahah