where males have (usually) a blunt rounded abdomen (some species have a long pointed end), but females have four obvious appendages folded together to make somethinf of a blunt point. These are collectively the ovipositor (at least the visible part of it). Sometimes it's pretty easy through secondary traits, like how long are the wings, how thick are the legs (front and middle femora are thickened in male Orphulella), etc. Often you can tell by familiarity with the insects and just be looking at the proportions (females in nearly all species are large and stockier than males). By the way, the one in the thumbnail is a male. :o)