Definitely the shave test...
I have some small lay flat whiskers at the bottom of my chin. When the blade doesn't cleanly slice those off going WTG, it is time to refresh the blade. If that doesn't work it is time to get out the stones.
I found early on I probably did the most blade damage with bad stropping. Now the stropping will materially improve the edge I have.
But definitely figuring out some place on your face where a sharp razor does the best and then paying close attention to how that area works? Best test!
Today I even go as far as switching to the rotation razor if the current one doesn't do those whiskers well, and then I do some blade maintenance. Definitely the best reason to have more than one SR.
Good luck!
Ed