In trying to identify some photographic subjects last year, I learned that damselflies mostly fold their wings when they alight, whereas dragonflies hold them straight out, like a biplane. There are, of course, other differences, but none quite so easily observed as the position of the wings at rest.
Their aquatic nymph stage is also quite distinct.
Damselflies have three leaf-like gills at the tip of their abdomen,
while the more robust dragonfly nymphs lack these.
This is less than entirely useful, since it is not the nymphs taking flight and landing prettily on my kayak or a nearby lily pad.
So my practice is to wait for the insect to land, and hey presto, at least I know which suborder of Odonata it falls into. Determining which of Ontario’s 27 species of dragonflies or 21 species of damsels – ah, that’s for another day and maybe for another photographer.