Your best protection will be to obtain a support order for the agreed amount. That way your ex-spouse can never claim the funds you pay are not for child support.
There are a couple of ways it can be done. A simple statement, notarized (you each keep an original) is easiest, but she can claim you didn't make the payments after you signed. Setting up a joint bank account that is titled "child support" where you make the deposits and she can withdraw can work, and show proof of exactly what you deposited. The best way is to have a child support order, so the state keeps the records and no one challenges them.