How would I diagnose a dry solder joint?
Sometimes you can see that the solder is no longer attaching the pin to the board, and that there's visible movement somewhere. Sometimes it just looks a bit dull and grey, rather than shiny, or that it's formed a blob rather than flowing across the surfaces its meant to be joining, although lead-free solder always looks a bit crap. If in doubt, it doesn't usually hurt to re-flow the joint with a bit of fresh solder.
On a bad day, the solder joint is fine, but the copper track breaks free from the board. Fixing that sort of thing tends to involve bodge-wire between the pin of the component and somewhere else on the track.
There are countless soldering videos on Youtube, so that's a good place to look for examples of this sort of thing.