You should look up some popping videos on Youtube. Ralph Davis has a couple of good ones. Or, have someone (like a breeder) show you how, or do it for you.

At this point, the male should have sperm plugs come out when you try to pop them. You don't even need to get the hemipenes out, just see if sperm plugs come out. It's a dead giveaway for a male...

Since you know the pastel is def. a male, try him first and see if sperm plugs come out. He is definitely big enough to have them. Then you can see what they look like.