Based on a single shift operation, forklift forks should be inspected once a year for wear and distortion. The most common way is to use a fork caliper, this adjustable "go-no go" tool is used to measure the thickness of the shank of the tine and compare to the blade portion. Cascade's model includes a reference point that will indicate if there has been more than 10% wear.
Additional inspection suggestions can be reviewed at You Tube link