usually in the grafic's cards drivers ( resolution propperties, and then the advanced button,.. there you should be able to select a tab called like your grafics card,)
im completely not famillair with the Ati drivers.. In the nvidia drivers there is a tab Temperatures, wich is quite obvious what you find there i think
Riva Tuner is for Nvidia, you could try ATI Tool, it is overclocking software but I use it because when I minimize it the temperature shows up on the lower right right taskbar. Convienient I think.