The statute of limitations "clock" starts ticking on the date of last activity on your account. Typically this is the date you last made payment, but it can also be the date you last made a promise to pay, entered a payment agreement, or even acknowledged liability for the debt.
Often, you can check your credit report for the last date of activity to figure out when the statute of limitations clock started. That's only if the last activity is a payment or charge to your account - activity that's reported to the credit bureaus. Verbal activity, like a payment arrangement or acknowledgment of the debt, is not reported to the credit bureaus, so only your notes or records (if you've kept them) can help you know for certain when the last activity was on the account.
You can ask the creditor or collector for the last date of activity on the account. While they're not required to give you an answer, they must answer truthfully if they decide to give you an answer. If you have doubts about the timing of the statute of limitations for your debt, contact an attorney.