In the days of "proper" solder made of 60/40 lead & tin, well made joints had a lovely smooth bright silvery appearance. If the joint was badly made, i.e moved while cooling, or too hot an iron having evaporated all the flux, which is what can happen when you carry a blob of solder to a joint rather than applying iron and solder to joint together. The "dry Joint" will have a crystaline dull look and be higher in resistance. As current flows through the joint, due to the higher resistance it will heat up until eventually it fails. Any component that runs hot in normal use like wire wound resistors, power transistors etc. plus those that carry any heavy mechanical load, especially transformers that vibrate, will exacerbate the problem.
Mechanical security first, then solder was always the order of the day. Not vital in a tv where a fault is an inconvenience, but dire in an aircraft's electronics!
Modern Lead free solder tends to look a little dull even when good joints are made so it comes down to experience a lot of the time.
Why is it called "dry joint"? Probably because a good old leaded solder joint still looked fluid when cooled but appeared "dried out when badly made.
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