Females of the UK’s two Demoiselle (Calopteryx) species can be very tricky to separate.
One feature, the wings being essentially clear, is their slight colour tint, brown in Beautiful Demoiselles versus green in Banded Demoiselles, but this is not completely reliable for two reasons. Firstly there are naturally occurring exceptions and secondly the apparent colour of the tint can be affected by surroundings.
The second feature is the position of the pseudo-pterostigma, a white wing spot, relative to the wing tip and wing node – further from the tip in Beautiful Demoiselle, closer in Banded Demoiselle. Ratios are published to make this a little more quantifiable. Unfortunately there is a small amount of overlap so this, too, may be inconclusive.
Thus, from a photograph, certainty could be virtually impossible but combining these two features should work most of the time.
Slipping into glorious Technicolour briefly to make potential use of the wing colour tint: