How do you know if your TT needle is worn out?

I have a older Sota Star Sapphire turntable with Sumiko MMT arm and Grado reference cartridge, how do you know when your needle is worn out?
Have someone with an experienced eye look at the stylus under a low-power microscope. Signs of wear will be easily apparent.
when it is really worn, you will heard a lot of distortion. Hopefully, you will have replaced the stylus before then.
Take a post it piece of paper. Place the stylus on the paper (lay the paper on the table mat, but do not turn anything on). Genltly turn the platter so that the stylus passes over the paper. If the stylus catches, or scrathes the paper, it probably needs replacing. It should go over the paper smoothly and not snag along.
Otherwise, take it to a dealer who can look at it under a scope.
Good luck.