Is there a way to get a first fix on the water level in a tractor's tires?

[MHarryE]

Rotating the tire is the most accurate if you are in a climate where you can't see the dew line in the right weather.

 

[Lorre]

A sharp pointed object would work also. Just punch a series of holes in the sidewall until water is noted running out instead of air.
The tyre will be useless, but at least you'll know how much water was in it.