How to tell the temperature of a video card
If you are concerned that your video card might be getting too hot, there are several methods for determining the temperature of the video card.
First though, be aware that video cards are designed to be able to run quite hot. Many video cards run at temperatures in the 60 to 70 degree Celsius range, and this is completely normal. Some higher end video cards can run cooler, due to a more powerful cooling fan or other cooling mechanisms, but most video cards have a basic heat sink to remove heat.
In most cases, there is no real need to be concerned about the temperature of the video card. However, if your video card does manage to get above 100 degrees Celsius in temperature, then it probably is time to find a way to cool it down or replace it. However, anything under 100 C is okay. If it consistently stays over 100 C, you should consider replacing it, to avoid any damage to the rest of the computer or the possibility of it starting a fire.
Note: Excess dust build up and lack of proper airflow within the computer case can cause heat issues with video cards, as well as computer processors.
There are two ways to determine how hot your video card is running. The most common method is through a software-based temperature monitoring system, and the other is by using hardware-based temperature probe.
NVIDIA Control Panel
In the case of NVIDIA video cards, some models of their video cards show temperature information within the NVIDIA Control Panel.
Third-party software tool
For other video cards, there are dozens of different third-party temperature monitoring software available that are capable of displaying the video card or GPU temperature. Typically these programs monitor other hardware as well, to provide a full picture of how your computer is doing (like a health check). In the picture below, is an example of CPUID HWMonitor a free to use utility for monitoring all temperatures of the computer.
On the hardware side, you can install a temperature probe which attaches to the video card directly. The other end of the probe attaches to a digital display console, which usually fits into a 3.5" or 5.25" drive bay. This console displays the temperature of the video card. These probes can also be used on computer processors and some higher end probes feature multiple probes for monitoring more than one device at a time. For example, with enough probes you could monitor the video card, processor, memory, and hard drive. In the picture shown to the right is an example of the Aero Cool Touch-2100 that is capable of monitoring up to five different devices and adjusting the speeds of five different fans.
Finally, there are also infrared temperature guns that are capable of determining the temperature of anything in the computer. A temperature gun can be pointed at the video card and the GPU heat sink to determine the temperature. Keep in mind that using this gun is only going to give you an estimated value since it cannot be directly pointed at the GPU with the heat sink attached.