A good place to start would be the National Student Loan Data System. That site will list all of your loans and who currently holds them. Although the DoE did terminate some collection contracts recently, it might take a while for all of the accounts to be moved. If you're concerned, you should certainly contact Pioneer and ask them for a breakdown of how the payments have been applied. If they don't respond or your have concerns, you can contact the Student Loan Ombudsman for help. I'm sure they'll be interested and verifying that everything is correct.
You should also do some reading about loan rehabilitation. If you rehab your loans, it can improve your credit, stop the garnishment, and once the loans are out of default, you have access to deferement, forbearance and income-driven payment plans again, which is a big plus for managing yor loans.
