Visible damage, change in speed, change in the noise it makes, chatttering where it didn't before. Hold on to the output arm (gently) while cycling the servo. Sometimes you can break a servo gear in the impact. After a major OOPS!, I send the whole shebang radio back, not just parts. One of the oddest acting post-crash radios I ever owned was suffering from a broken receiver crystal. Worked with engine running, acted up with the engine off. Go figure.