Between the miles traveled, the weight they're carrying and exposure to the elements, your trailer's tires can take a beating. Before you hitch that trailer up and head out, it's important to check its tires over thoroughly. Boating Magazine recommends that you:

Check tire pressure. Make sure the tires are filled to the recommended air pressure, which is typically noted on the tire's sidewall. If the pressure is too low, it may lead to tire failure due to excessive flexing and heat buildup. Salt Water Sportsman says you should also check the pressure on spare tires.
Inspect tire tread and sidewalls. If you notice bulges, separation or cracks, it's time to replace the tire. You should also look for dry rot, says Salt Water Sportsman.
Have the tires balanced. To help prevent uneven wear and to keep the tires moving smoothly, get them balanced regularly.