Part of my Support payment in NY go to child care. How do I prove that a Baby Sitter no longer exists. Can I hire a P.I.

I know for a fact that my over $200 payment for child care alone is no longer going to a sitter. 6 y.o.twins are in school now and her 18yr child picks them up till parent comes in. How can I prove this w/o her lying in court and bringing in anyone as a sitter. Also, the $500/wk total payment left me broke 4 years ago so I've tortured myself with OT to survive and now I probably make almost double from back then. If I go to court to fight the child care payments it might hurt me in the long run if they re-calculate payments with my new income with all the OT. Also, can I prove no sitter existed from Sept. and recieve arrears to off-set arrears that they are saying I also owe, which is a whole other false calculation I want to fight.