Measure the distances from the center of the hitch to the ground and from the center of the connecting pin to the ground. The difference between this two measurements should be no more than three inches. Less is better.
(That comes from both Blue Ox and Roadmaster manuals.)
Tom
2015 Meridian 36M
2006 CR-V toad
3 golden retrievers (Breeze, Jinks, Razz)
1 border collie (Boogie)