The wheel bearings must be checked, lubricated or replaced at least annually. Before storage or standing for a long period is best, but if not done then it must be done before using again.
Check the tyre condition and inflation
Check the braking system linkages are free and lubricated.
Check the trailer lights are all operational.
Check the boat hold down straps are in good condition and firm.
Service the winch, lubricate and ensure working parts are free and the wire rope in good condition.