One question I would have is the tractor sitting on a level surface.? If not the fluid could easily be tilted over the valve stem.. I usually shoot a little air in first before I try to check the pressure to clear the stem, and then you can gauge the pressure with a standard gauge, providing that the tire was not filled too full to begin with. But in any case if it looks low, NOW is the time to put some air in before the bead comes loose from low pressure and dumps your load of fluid. A lot of folks dont bother with a gauge anyway, they just add air until the tires tread touches all the way across. But before I started to let fluid out, I would add air if I felt the tires were low to mitigate the risk of bead breaking. Of course buying a gauge designed for liquid is an excellent idea.
