The Vagrant Darter (Sympetrum vulgatum) is very similar to the Common Darter (Sympetrum striolatum) and the Ruddy Darter (Sympetrum sanguinem).
However, the Ruddy Darter can be easily recognized by its completely black legs.
The other two Darters have yellow or brown stripes along the legs.
The Vagrant Darter can be distinguished from the Common Darter by a black line on the forehead that runs downwards along the eyes' edges (like a moustache).
In addition, the body of the Vagrant Darter, especially that of the male, is slightly shaped like a bludgeon (not straight like in the Common Darter) and the ovipositor of the female sticks out in a 90 degrees angle (where the ovipositor of the female Common Darter sticks out in a 40 degrees angle).

Karsiborska Kepa Nature Reserve, Poland, 17 August 2012