Under normal operating conditions, forks should be inspected daily and every six months.
Daily: Operators should make visual inspection of forks during the pre-start-up check, giving special attention in looking for permanent distortions and cracks.
Six months: A thorough inspection of forks should be done every six months, preferably by a trained individual, to check for any cracks and distortion. Forks may need inspections more often, depending on the use of the equipment.
Check with your jurisdiction for any requirements regarding frequency of checks, and maintenance.