First, lesser stag beetles are not as easy to sex as stag beetles are, but it can be done.
They are smaller than stag beetles, their size could vary from 18 to 32 mm.
Another problem is that they could easily be confused with a female stag beetle. However the size, and colour of the wing cases should be sufficient to distinguish the two species. The stag beetles have a slightly lighter colour on their wing wing cases, whereas the lesser stags have all body parts the same black colour.
Thirdly, the lesser stag beetles have ridges running along the the middle section (tibia) of their front legs. So to make double sure you must also check their legs.
Another thing, they are remarkably similar to the antelope beetles (Dorcus parallelus) a north American species. Just have a look at these pictures as they might help you sex your lesser stag beetle.
I hope that this page has helped you. Don't hesitate in giving me your feedback as I have had a lot of trouble learning how to sex these beetles. Just read Stag beetles in my garden if you want know about some of my mistakes.