Elliot Savitzky replies: If you are having to push the wood for a rip cut more than usual, you are getting burn on wood that you are not used to and/or if you are getting tear out or chipping, it is probably time to sharpen. Look at the blade and check for chipped or rounded teeth—they reflect the light differently. You can start by cleaning the blades and see if that helps before sharpening or buying a new blade.
Richard Oedel replies: First, you should always have a second, sharp blade for your table saw. If you think it is getting dull, cut a piece of scrap wood about 20˝ long, change the blade to the sharp one and try it. If dull, the difference will be remarkable and only takes about two minutes of your time.
Jon Siegel replies: Sometimes you can detect a dull blade by close visual inspection of the teeth. A dull edge will reflect light as a bright line. This method is somewhat subjective and may not be the best way to determine sharpness. Below are some of the functional symptoms of a dull blade:
Often a blade will show some of these symptoms, but the only thing wrong is that pitch has built up around the teeth. Remove the pitch and you might be surprised at the improvement.
Steve Costain replies: Get yourself a good 10x or 20x eye loupe and compare new to old. At that level of magnification you can see the edge.
Garrett Hack replies: It will take greater force to push stock into the cut and you’ll smell and see some burning. Don’t let your blades get this dull for your own safety.
Bob Couch replies: Burning and more effort to push the material through as compared to another sharp blade you may have. Also motor lagging or popping the breaker are also signs.