What proof so I need to look for when a debt collection agency is taking me to court

Always ask them for their proof since they are the ones that have the burden of proof if they are taking you to court and you are the defendant/ debtor.  If they cannot provide proof of a valid legal agreement that is signed by you or a valid signed judgment against you, then they will likely be unable to collect a debt from you.  A creditor must prove nonpayment or breach of the terms of the valid agreement to pay and provide an accounting showing how and when there was a default of that agreement, when and how you were notified of that default, and whether, how and when they gave you an opportunity to cure the alleged default.  All of this proof can be found via discovery requests for all account ledgers, correspondence, internal notes, phone logs, certified mail receipts, and all signed promissory notes, agreements, assignments of debt etc.