Is there a way to check the extent of brain damage caused from drug use?

Yes and no....: Early on i doubt any changes would be detectable. However, with extensive use structural changes may be seen. In my experience, drug users show loss of brain volume at an earlier age than otherwise. Functional MRI and pet on the other hand would show changes sooner. That is, the relative activity of certain centers of the brain would change without causing structural changes like volume loss.