How do you check the hydraulic oil in a tractor?

This all depends on which tractor you're on at the time.

The older Fords 8N's - Jubilee's - etc. had a dip stick on the side of the transmission.

A lot of the older International's - you needed to take a twig and stick it down the reservoir.

Most have a top off hole. Meaning that you add fluid until it runs out of the top off hole. Then you put the plug in and the fill cover back on and it's full.

Go to tractorshed.com for specific help on your model.

The location of the hydraulic oil check depends on your make, model.