How to tell the sex of two red neck slidder turtles?

Answers:
Both of the above are correct. You can also look at its cloaca (opening on its tail). Males have a cloaca that is further towards the tip (and away from the body), while the opening is close to the body in females. As previously mentioned, these traits may not be readily apparent if the turtles are less than 5 years old.

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see if one ever pops a ,,,,,,,

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It's hard when they're young, but when they get older, the males will have concave bellys (inward sloping), females will be flat. This is to assist the mating process. Males also have longer claws, which is obvious in a side-by-side comparison. I would search this topic on Yahoo or Google in order to see some photos.
Also, it's 'red eared slider', not 'red neck slidder' =P

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I read online that females tend to be bigger, but male ones will have longer tails.