You can do an optical bench test using either Foucault knife edge or Ronchi grating. You will likely have to remove secondary and/or spider so that the light source is not obscured.
The best method though is a star test and there are plenty of resources online as well as this book:
http://www.willbell.com/tm/tm5.htm
If you want a test with a laser interferometer then I think the mirror would have to be removed.