Lots of resources at etrailer.com. Diagnostic tools, instruction videos.
With a cheap (sometimes its the free item) multitester from Harbor Freight, you can test the voltage of each pin on the truck, and do a continuity check from each pin on the trailer side.
HFT also sells a little gizmo for under $10 that you plug into your truck and it tells you which is working or not. The catch is they only have if for a 4-way flat connector so it won't help diagnose a 6-way or 7-way. Actually, they also have an adapter for a 7-way that you can use to connect a trailer with a 4-way, and you could use that with the tester.
But really, you need to know how to use a tester anyway, to make the repair.
A lot depends on how the trailer is wired, as well as how the truck is wired.
Also, something something something LED lights different something.