How can I detect the speed of the user's graphics card so that I can decide whether or not to enable advanced graphics features that would slow down the app on a slow card, such as anti-aliasing? I am using OpenGL.
Usually you would just leave this up to the user to decide or alternately run a small bench test and recommend settings to the user.
If you want to bench test to recommend settings or just put it on regardless all you need to do is see how long it takes to complete your render call. If a frame is 25 milliseconds, for example, and all your draws are done in 12 milliseconds, then you can probably turn it on.