Sometimes you can look at them and see uneven wear on them. Do you rotate them at all?
I am a larger user, so I am constantly checking them and moving them, so the wear is not always on the same spot. I have several different strengths, so I swap them out frequently. When I start "coning" (hitting the floor when I'm bouncing) I know I either put on a lot of weight or they need replacing.
Just look at them. If you do a search on You Tube there are several that show how to rotate them. (at quick glance i couldn't see if they have anything on the wear of them.) But truly - you can see where they start to wear through!!!!
If you purchased it used, use it regularly and are questioning if it's time??? It probably is!!!
Good luck.
Cleda