Note that the use of a glass heater can be dangerous. If the glass breaks, both you and your turtle may be electrocuted. Turtles can be vigorous - be sure to protect the heater from the turtle. The operation of the under-gravel filter will circulate water around the heater. If there is not enough gravel to protect the heater, consider surrounding it with bricks or stones.
Some brands have a heater guard that fits around the glass tube and provides some protection.
There are several brands of heaters available. The Ebo Jager heater is considered to be among the best. This last feature is very important - cheap heaters using thin glass tubes will not withstand the onslaught of a vigorous turtle. The Ebo Jager has its temperature setting indicated on the adjustment knob, but you should always check the temperature with a high quality aquarium thermometer, at least until you get the setting correct. Hagen has the Hagen Tronic series that also can be recommended.