How do you test an OpenGL application on older GPUs?

I do all of my development on a fairly up-to-date machine, but some of my users use machines from the stone age. It's easy enough to virtualize older operating systems and different CPU architectures, but I haven't found any way to virtualize older GPUs.

When you're developing a program targeting an older version of OpenGL, how do you know if it will work on machines with older GPUs (lower OpenGL versions and fewer available extensions)?

Are there GPU emulators? Ways to turn off certain features on your GPU?