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Look under their tail, right where the tail meets the body. If they have 2 little lumps, then they're a boy. If they have no lumps, or just a single lump. Then it's female.
Monkey is right, the dark on the body means they are very stressed. He is not happy with something in his tank or how he is being handled. Did you get a probe temperature gauge to check the temperature right at the basking spot? It should be around 110 degrees. The cool side should be 80. Is he eating his live food? Is he pooping?