First, you gotta pick it up. Feel its weight. A ripe lemon should feel heavy for its size. Why? Because juice is heavy. A light lemon is a sad, dry lemon. Give it a gentle squeeze—not a death grip, just a little poke. It should yield slightly, like a firm handshake, not a limp fish.
If it’s hard as a baseball, it’s underripe. Think of it as the lemon equivalent of a green banana. Nobody wants that in their tea. If it’s mushy? Run. That lemon is on its deathbed.