What all documents might they possibly ask for as proof of work ex? Is app. letter and relieving letter sufficient or do they ask for salary slips, PF etc. as well? Also, do some colleges call up the employer to double check?

Salary slips can be asked as well. PF will not be asked. They might email/call your employer, and as far as I know during your internships and placements they are almost certainly going to at the very least get email verification from your past employer

Okay. Thanks a lot for the help, appreciate it.