It is often said that mature males of Emerald Damselfly (Lestes sponsa) and Scarce Emerald Damselfly (Lestes dryas) can be distinguished by the extent of their pruinosity on the dorsal side of S2:
the basal half of S2 on L. dryas is clear dorsally whilst the basal half of S2 on L. sponsa is pruinose.
This is only partially true in that the basal half of S2 on L. sponsa can also appear clear. [See below] It does tend to look cleaner in L. dryas, though.
The partially true bit is that if you do see a pruinose basal half of S2 then it will be L. sponsa. If the basal half of S2 is not pruinose, you may be uncertain.