Do you have copies of your credit reports? If so, read to see if it says the account has been sold. If so, you don't need to deal with that one.
They are not going to send you any proof of anything. Eventually they will send you a court summons. That is the only time you will be able to make them prove anything and it is an ordeal.
Do you have old statements? If so, they will have the account number. Try to track the account numbers on the credit reports.
When you talk to a creditor ask the following questions.
Who is the original creditor?
What is the date of last activity?
Has the account been charged off? If so, when was it charged off?
Are you the original creditor? If not, who owns the debt today?
The answers to those questions should help you figure out what is what.
Are we talking about a lot of money here?