The Common Darter (Sympetrum striolatum) is very similar to the Vagrant Darter (Sympetrum vulgatum) 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 Common Darter can be distinguished from the Vagrant Darter by not having a black line on the forehead that runs downwards along the eyes' edges (so no moustache).
In addition, the body of the Common Darter is straight (not shaped like a bludgeon) and the female's ovipositor sticks out in a 40 degrees angle (where the ovipositor of the female Vagrant Darter sticks out in a 90 degrees angle).

1st, 2nd & 3rd photo: Neuenburger Urwald Nature Reserve, Zetel, Wilhelmshaven area, Lower Saxony, Germany, 15 August 2023
4th photo:  Lake Pütten, Budjadingen, Wilhelmshaven area, Lower Saxony, Germany, 16 August 2023