There are several types of relays but the most common are the little ice-cube relays. About 1.5" square and have 4 terminals. I'll assume you are talking about them.
The terminals are usually labled 85, 86, 87, and 30.
85 and 86 is the control side of the relay and create a magnetic field to close the "switch" between terinals 87 and 30.
The easiest way to test is to check for continuity between 87 and 30. There sould be none, or the switch open.
Then add 12v to 85 and ground 86 and see it the continuity between 87 and 30 goes to 0 ohms (switch closes).
Some relays have an 87a terminal which is a normally closed terminal. meaning it has power (continuity with 30) without 85 and 86 having power and ground, and opens (no continuity) when power and groung are applied to 85 and 86.
Additionally, some relays are controlled with the ground wire on 86 and otherswith the 12v on 85. Depending on your tractor, it may have 12v all the time on 85 and when you what to enetgize the relay, in closes the switch and allows 86 to ground. Others have 86 grounded all the time and when the relay needs to be energized, it supplies 12v to 85.