Red Eared Slider

Yea, with most it helps to just sit near them and be still for a few minutes. Most will peek out when they don't feel threatened anymore. I'd be willing to bet it's a Red-eared Slider, Trachemys scripta elegans, they are very common and patterning looks right, but many turtles can have pretty variable patterning. Here's a link for you http://www.louisianaherps.com/pond-slider-trachemys-scrip.html

Pretty hard to ID one of the pond turtles just from the shell, helps a lot to see the patterns on the face and arms.