Of the four UK Emerald/Spreadwing species, the females of two can be particularly tricky to distinguish: the Emerald Damselfly/Common Spreadwing (Lestes sponsa) and the Scarce Emerald Damselfly/Robust Spreadwing (Lestes dryas). So, here’s my attempt to clarify the significant distinguishing features.
One feature prevalent in most currently available documentation is, IMO, particularly difficult and seems variable; this is the issue of the length of the ovipositor. Omitting this trait from such a comparison would look like an oversight so I indicate it with a warning. There is a slight visible difference in my examples but I think it is unreliable.
Another feature, rather difficult to diagram, is the stoutness of the build, heavier in the appropriate alternative name for L. dryas, Robust Spreadwing.