You did not specify the type of debt, but generally, the creditor will provide evidence of the contract (lease agreement, credit card documents etc.) they will show previous payments, current balances, past due amounts, fees, late charges etc. They may document their efforts to contact you (copies of the various letters they sent and your responses (if any)). The standard in most states will be “preponderance of the evidence” (i.e. it’s more likely than not; or 51%)
If you are attempting to defend yourself, against a collection agency or its attorney--you are at a serious disadvantage and should consider retaining an attorney immediately. If you cannot afford one, many state or county's offer free legal aid--use Google and find a local agency that can help you.
In MANY instances, especially for credit cards, the creditor cannot account for bills and balances from previous years because they creditor purchased the account (banks buying credit cards etc) and as a result they do not physically have all the documents they SHOULD need to get a full judgment against. A competent attorney will know how to highlight such a discrepancy and you may be able to get significant reduction in the total amount the creditor will accept in lieu of full payment.
I am licensed only in the Commonwealth of Virginia, I try to provide sound guidance but I am not providing legal advice, you are urged to contact an attorney in your home state for specific guidance and legal advice. We do not have an attorney-client relationship and I am under no obligation to respond to subsequent emails or phone calls. I wish you the best of luck.
Usually they will have the record custodian submit an affidavit that they are the rightful owners of the debt, that the billing statments submitted to the court are true and correct, and the amount owed is X.
The general advice above does not constitute an attorney-client relationship: you haven't hired me or given me confidential information by posting on this public forum, and my answer on this public forum does not constitute attorney-client advice.