Here's how we did toe-in checks on truck steer axles; jack up wheels, clean off center of tires
, spin wheels while spray painting
center of tire with white paint
. When paint
dries spin wheel again with a pointed device against center of tire to make a mark around tire, remove jacks, drive truck forward 10 feet or so to set suspension in it's normal travel mode, measure between scribe marks at front & rear of tire to get toe-in measurement. Another method we later used was a piece of aluminum channel hung on the outside of the tires
, hook a tape measure at front & rear of tire on to the one channel, read measurement at the channel on the other tire. On tandem axle trailers we had devices that went in the wheel and centered on the spindle, then measured between them to check the axle spacing. Used this same device to measure from the forward axle to the trailer kingpin, or in your case the center of the ball coupler. First step would be to measure front axle to ball to verify it being straight, then check the spacing if it's a tandem.