How do you know a game is backwards compatible for the 360

Here is the address to the list of games that are backwards compatible (just copy and paste it into your address bar at the top of your browser and hit "enter"):

http://www.xbox.com/en-US/games/backwardcompatibilitygameslist.htm

You must have a hard drive on your Xbox 360 so that you can download the backwards compatibility updates either through connecting to Xbox live, or by burning the updates on a CD and putting them in the CD drive to be burned to your hard drive.

Instructions on downloading the backwards compatibility updates on your 360 are here:

http://www.xbox.com/en-US/games/backwardscompatibility.htm