The best way to find out is to go to the National Student Loan Data System website provided by the U.S. Department of Education. Follow the instructions to set up an account, and when you get access, any loan activity you see there deals with federally guaranteed loans.
Any other student loans you have are private loans.
One difference is that federally-guaranteed loans can be collected against you without regard to how old they are (no statute of limitations) and can be collected from your paycheck by means of an Administrative Wage Garnishment Order, no lawsuit or judgment necessary.
For private student loans, statute of limitations defenses DO apply, meaning that the lender has a limited period of time to file a lawsuit against you after you went into default. The amount of time depends on the applicable state law for enforcement of contracts.
Go to NSLDS website to find out which you have and to plan accordingly.