You can tell if the card is running to hot because you would start to see anomolies on the screen (like odd patches of colour or black lines- strange stuff).
So if your worried run a graphically intensive program like 3dmark in a loop for a while and if the images show no signs of fault then you are prolly fine.
As Manfordjinsen says video cards run hotter so I think your temps are fine, but the is nothing like peace of mind