The teeth on the nose sprocket will be very pointy, the chain will have alot of slop side to side and wont cut straight. The bar rails will be worn enough the chain bottoms out in the groove. There will be wear in front and behind (Depending on which way the bar is flippped) of the nose sprocket. These are all ways to tell your bar is getting bagged out. If the grove is worn badly you can run the next bigger gauge chain in the bar to get a little extra life from the bar.