The rear tires need to be slipping before the differential will lock. Normally, one tire will spin, the other will not be turning.
When you use the diff lock, both will be turning, and hopefully, you'll be moving.
I never "feel" it lock...just see both tires turning and the tractor (normally) moves ahead...
BTW, this a for low speed operation only, and you should be going straight. Once those tires start turning and you're moving, remove your foot from the pedal. It should remain locked....once traction is regained, it will release.
I don't know of any good way to test this unless you're in a slippery, low traction situation that would require use of the differential lock.
We do have a TBNer, Soundguy, who is very knowledgable about these older Fords. You could PM with your question...