What are some acceptable form of proof that I have a college degree?

No, they don't. But if they need proof they can contact your college. {You can make copies.]

Usually employers don't require proof of degree in the selection and interview process, only just before they make a commitment to hire you. 

They'll want a transcript, if anything, not a copy of your diploma. You can have an official transcript sent directly from your university to the employer. 

However, nine times out of ten, these days, they'll just get the information from the National Student Clearinghouse, which provides degree verification and enrollment verification. 

Official transcripts can be sent directly from your school to an employer. 

You can purchase an official transcript  and have it sent directly freom the college to an employer.

A transcript and or just the info on when and where you gradate.  Educational verification doesn't work the way you think.  Employers don't care about your diploma.  If they care about whether or not you graduated and what your major was, they will contact the registrar's office at the university for verification.  This happens every day.  Many universities have a way to submit that request online.  Some employers may ask to see a transcript, which is also something that has to be requested from the the university.

They dont care about the diploma (and yes you photocopy it and can give it to them as a start and I advise making many copies to have); they CALL the uni and ask for formal verification. The diploma is just a piece of paper and even the original is not considered "proof" as they can be made up. A few jobs I worked at even requested sealed transcripts but that is pretty rare.