All debt collectors are required by law to send you a DEBT VALIDATION. This is sent through the USPS ONLY (email, text, faxes, carrier pigeons, smoke signals...none of those count) and state when you incurred the debt, to whom you owe the original debt, and how to proceed should you not recognize the amount.
This must be sent within 5 days of the first time the company attempts to contact you.
REAL DEBT COLLECTORS SEND THIS. Scammers do not.
A few other things to know-
-You cannot be arrested for ordinary debt in the United States.
-Process servers do not call in advance. They only get paid if they deliver you papers the court (not a private company) has ordered be delivered, and if you flee, they don't get paid. Therefore, they do not call ahead of their arrival.
-Likewise, officers do not call you to tell you they will be arresting you. Why would they?
http://www.consumer.ftc.gov/articles/0149-debt-collection
This link contains all the rules debt collectors must follow, and includes links where you can report criminals who choose not to follow the law