I need proof of vaccinations in order to attend college. However, I can not verify my rubella vaccine. What can I do?

Titers. If you do not have documentation of your rubella vaccination, you can have a blood test done to determine your "titers". This will show if you have adequate levels of antibodies against rubella and prove that you are immune. Colleges will usually accept this as proof of immunity.
Get the shot. You may or may not have blood titers that show you protected but the study usually takes a while to come back. You can simply get another vaccination. There is a higher frequency of some transient joint pain as a side effect in young adults, but the benefit is lifelong.
Check blood titers. Check a blood test to determine your immune globulins that show whether you are immune or not.