Look and snap
A well tempered bar is glossy and breaks with a clean snap. If it bends or crumbles, it got warm on the way. It will still taste good.
Smell first
Break a piece and hold it under your nose. You will notice more than you expect: fruit, roast, wood or flowers.
Let it melt
Put a piece on your tongue and do not chew. The flavour changes as it melts: the first notes are often fruity, the middle more roasted, the end bitter or nutty.
Wait for the finish
After it has gone, wait. A good bar keeps tasting for a minute or more. That long finish is the sign of good fermenting.
Compare two
Taste Ridge Dark 72% next to Palm Sugar Milk 48%. Same beans, different recipe. The differences teach you more than any description.
The best tasting tool is a second bar.