Males will have longer tails than females, with the cloaca nearer the tip than the base, females have much shorter tails with the cloaca much closer to the base. If you have males and females the difference should be quite obvious.

Whilst females might be more docile than males, be aware that they can grow much bigger. If it's an Apalone species the females can grow to 18" SCL, males around half that, personally I'd prefer a snappy 9" male!

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Sorry for my late reply. It's difficult to tell, males of that species appear to be a bit smaller, but it's not for sure. The best way to know it is... well, it's a bit disgusting, hehe. Before I knew Frank was a male, I used to call him "Lola" -blush- until one day when I saw something while I was cleaning him. I picked him up and put him under the water. A kind of a penis came out. If your turtles don't bite you or are not nervous (moving, and so they could bite you), try to do that. Generally, males (as they keep the penis hidden in the same place where the anus is) have a longer and a bit swollen tail. Females' are smaller. So first have a look, and if you don't have it clear, put them under the water.
The only problem is that if ur turtles are younger than 6-7 months, perhaps u won't know it...
I hope I have helped you

You can also see it if they're swimming around, as males (as far as I know) like showing their "friends" while they swim.
I agree with Graham, but I sincerely prefer a more docile female. Anyways you said that they could also be spiniferas no?
I'd love to see a proper pic of them, so that I can tell you. Spiniferas are not agressive with people, only a bit between them. In that case you could keep a male perfectly.

Wait... those black spots... guess they're spinifera.