You basically have four wires coming from the truck (plus a possible fifth for the reverse solenoid).
Listed in no particular order
1) Common ground
2) Running lights, side markers, etc. all on one wire
3) Left side high intensity only (brake & turn)
4) Right side high intensity only (brake & turn)
If your truck lead for the passenger's side high isn't good, your trailer will have everything but the right side brake and right turn. It would be good to make sure the truck wire was good before you started in on the trailer. It might save some head scratching. Turn your right turn indicator on and use a test light to see if you can find a blinking hot wire at the plug.
If you don't have a multimeter, you can use a tester like this:
You could also pick up one of these easy truck testers:
Some adapters have lights as well: