A towing vehicle with a solid ball with lock washer and nut is proper and legal in all states.

Make sure the ball hitch lever is properly latched.

Safety chains are to be crossed under the tongue and latched to the towing vehicle frame.

Be sure the tilt bed properly latched.

Check wheel bearings for proper lubrication.

Check tire pressure (proper PSI).

Check lug nuts. (Are they tight or are some missing?)

Is there a spare trailer tire?

Some trailers have brakes (electric or hydraulic). Do they work?

Lights are required by law: left and right turn, brake and taillights, and license plate. Do they work?

Make sure the snowmobile(s) are secured to the trailer: metal clamps or ski rods in front, chain with load tightener in the rear.