DTB's example is of EXTREME lean conditions. Less lean can still be too lean though.

Try the "pinch" test. With the engine running full throttle, (carefully) squeeze and hold the fuel line feeding the carb. If the engine speeds up a little, then quits, it's set about right. If it just quits, it's too lean.

The other test involves lifting the nose of the airplane to nearly vertical with engine running full throttle. The engine should again speed up a little when the nose is raised, then drop back down when you level the plane. If you can't hear the engine speed up, it's too lean.
Dennis-