It’s really something that takes a little time to get a feel for. At some point, you will realize you have to push much harder than you used to for something as simple as a ripping operation.
You will also notice more burning on woods that didn’t burn before. In addition, you should notice more tearout on your cuts. If you don’t notice any of these changes, or they just aren’t that severe yet, then the blade is still sharp. The first time you notice these things, clean the blade. After 2-3 cleanings it’s time for a resharpening.