How do you tell a girl red ear slider turtle from a boy red ear slider turtle?

While it is difficult to determine, it's not impossible. Boys have longer claws and tails, which becomes more apparent as they get over 1 year old. I have a 1 1/2 yr old male and I noticed his nails had a growth spurt 2 months ago and are now very pronounced. My advice is to go to the pet store and ask for the turtles ages, then look at the different lengths in both the nails and tails. Good luck.