Mature male Common Darters (Sympetrum striolatum), being orangey-red, are mostly quite obvious, though beware older females which can develop a red flush.
The main confusion potential is between females and immature males, yet to turn red. The most definitive distinction is in the shape of the anal appendages/genitalia but, there are other pattern differences which can prove useful depending upon the view and clarity.
The points made here for the shapes of the appendages and genitalia hold for other darters; indeed they hold for other Libellulids. Note, however, that the prominence of the female vulvar scale can vary somewhat. In particular, Ruddy Darter females do not have a prominent vulvar scale.
Coloration is best ignored; older females darken with age and in some cases can develop a male-like red flush.