Tank damp all over or wet at one spot?

What clues help distinguish condensation on a toilet tank from a slow leak at a fitting or seam? Broad moisture over the tank seems different from a wet trail that starts at one joint, but I’m not sure how reliable that distinction is.

Does the tank drying between flushes tell you much, or can either cause leave it damp? I’d also like to know which simple checks are useful without tightening fittings enough to damage them.