The easiest way is to get a video card with this feature built-in. The software for checking this is included on the driver CD. If you didn't get one with a built-in temp sensor, then you have two options.
1) - Get an IR laser gun and "shoot" it at your video card whenever you want to check its temp. This means you need the case open each time you do this. The advantage is it doesn't void your video card's warranty.
2) - Install your own temp sensor using a third-party kit (these are usually used for monitoring CPU temps). This is probably about the same cost as an IR gun, maybe less. The downside is you void your video card warranty because you have to remove the cooling unit to install the sensor.
Have I missed any options?