You take a nice new one out of the package and put it on and marvel at the lack of noise as it glides through the wood like butter, instead of chewing through it like an angry gopher...
when the light reflects of the cutting edge, if you can see a definite edge or flat its dull.. it should be a very fine line that wont reflect light for us to see.
when you have to lean into the board to get it to burn its way through all the while tripping the smoke detectors in the house
'member the days before carbide tips? when you could just break out a number 10 mill file and touch up the edge of the stamped steel blade?
(of course that was every third board but... what the hay?)
I'm going to wait to see if dad tells about his bandsaw blade that was "just fine" till he put a new one on . . .
Thanks to many of the fine folks here I have learned that using dull cutters of any kind (that includes sandpaper) is a false economy. Your not saving a dime but trying to stretch the usefulness of your abrasives or delaying a trip to the sharpening shop. Wood is expensive. Treat it to sharp cutters.
When you're cutting parts for your toy project and your spouse says to make sure you don't go to fast that it smokes...but it still smokes no matter how slow you go