My ex has been taking me to court over the years claiming he is not making enough money and has been successful in reduced payments. His income has been garnished and he has been in arrears up until recently.
The Division of Child Support Enforcement has just started receiving big payments from somewhere and his arrears have disappeared (grateful).

My question is how do I find out the source of this new influx of money.

Without knowing the source, I can't determine if I should go back to court for an adjustment back to his original higher payments before he cried poverty or should I just be happy with what I got because it is a temporary situation such as tax refund.

You would have to take him back into Court and have him testify.

Otherwise this is private info -

And I just went through this with a family member, also in NY.

Unfortunately he lies in court, doesn't bring in any of his income papers.

Can the judge search the source of that income?

Why would a Judge be interested? He owes you nothing in arrears and apparently his current support is just that - current.

No Judge is going to care.

you hire a PI, have him followed, find out where he works, so you can get a court order for his pay records.

Have the PI research where he lives, how much his mortgage is and what types of car he drives, in court make him prove his life style

Have you sent him financial interrogatories to him in the past that must be answered in writing and notarized?

You can request a subpeona for his tax records.