For around $13 you can have a 7-way tester plug like this:
7 Way Trailer Connector Vehicle Tester TR20117 : Trailer hitch bike rack and trailer hitches - etrailer.com
That's a great start. If no lights at all on tester = ground problem. Otherwise each light will tell you what's not working.
Also, I find the 07+ tundra receptacle tends to get dirty despite the spring-loaded protective cover. I have to knock the dust off the leads by rattling the trailer plug while inserting in and out a few times to clean things up.