Jack wheels up, try rocking the wheels top and bottom to check for free play , a little is ok alot means bearings need adjusting or are on there way out, spin the wheels and listen for any noise from the bearings , any growling type noise = bearings f****d..
If its on indespention suspension check the rubber bushes ain't perished, if its on steel suspension make sure no leafs are broke , check the rest of the trailer for signs of rust or rot.. Check lights ....