How do you know when shellac is too old?

Shellac can be considered too old if it becomes thick, stringy, or has a strong, unpleasant odor. Additionally, if it doesn't dissolve properly in alcohol or forms clumps when mixed, it's a sign that the shellac has degraded. Always check the expiration date on the packaging, and if you're uncertain, it's best to test it on a small area before use.