How can you tell if there is anti-freeze leaking into the automatic transmission?
The only way that can happen is if the tranny cooler in the radiator developes a leak,.
Transmission fluid will start to look very cloudy and turn to goop, or look like a strawberry milkshake.
A radiator shop could pressure test the xmission cooling line. Either it leaks or it doesn’t. Those shops have the expertise and tooling and do it all the time. And they could fix or replace if there is a leak.
Take off trans cooler line from radiator. Pressurize cooling system. Does anything come out of fitting?
You could also have a small amount of the tranny fluid tested for the presence of coolant…and vice versa…at any test lab. It isn;t expensive, and it is definitive.