How do you know if your video card is going out on your pc?

Our computer keeps freezing. At first it was just on the net so I thought it was my ISP or something but now it does it everywhere. I will be trying to watch any video clip and it just stops and won't go on. I will be trying to play a game and it either completely freezes or just goes away. Just trying to figure out what is wrong? Any help is appreciated.