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).

1st photo: In my yard, Hilversum, The Netherlands, 23 July 2012
2nd photo: Stulpheide, Lage Vuursche, The Netherlands, 4 August 2014
3rd photo: Laarder Wasmeer, Hilversum, The Netherlands, 9 August 2014
4th photo: 's Graveland, The Netherlands, 11 August 2012
5th photo: Fochtelöerveen, The Netherlands, 25 September 2014