Before a session, check that each wheel spins freely and that no axle nut has worked loose. After a wet or dusty session, wipe the wheels and inspect the bearing area.
Every few sessions
Listen for new grinding, check for side-to-side movement and add a small amount of lubricant only when the bearing is clean and dry.
Replace before failure
Keep a spare set in a dry place. Replacing a rough set early is cheaper and safer than waiting for a bearing to seize during a session.