After coupling, how do you verify the trailer is securely connected to the tractor?

The 'tug test' is the verification: with the trailer brakes set, put the tractor in low gear and pull forward gently against the locked trailer. If the kingpin is properly engaged in the fifth wheel jaws, the tractor will not separate. If the connection is incomplete (a high hitch where the kingpin sits on top of the jaws instead of inside them, for example), the tractor will pull out from under the trailer and the trailer will drop catastrophically. This test must be performed every coupling, no exceptions. After the tug test, visually inspect the fifth wheel from underneath using a flashlight: confirm the kingpin is fully seated in the jaws, the locking lever is in the locked position, and there is no gap between the fifth wheel and the trailer apron.