How do you identify when tools need re-sharpening?

To some extent it depends on the tool:

• carving knife, ie, the type of knife used for slicing meats: If you can see the edge of the knife, where it should taper to a very thin almost invisible line then the knife needs sharpening.
• chain saw: If the chips ejected from the cut are becoming very small like sawdust or if the chain is producing smoke then the chain should be sharpened.
• wood chisel: If the chisel glides over the wood rather than biting in then it probably needs to be cleaned or sharpened.