I'm not that familiar with the Palearctic fauna, but have found that wing venation is a pretty reliable (say 95% of the time) way for recognizing Nearctic eulophid subfamilies. Eulophinae has the basic ground plan, with fairly long stigmal and postmarginal veins, and the stigmal vein is at an acute angle to the wing edge. Tetrastichinae usually have no (or really short) postmarginal veins (and most have 2 deep longitudinal sutures on the scutellum). Entedoninae have very short, equidistant stigmal and postmarginal veins, and the stigmal veins tend to be at a less acute angle (additionally their forewings are often distinctly wedged-shaped). Euderinae have venation like Entedoninae, but they have a space just behind the marginal vein that is bare of the usual short setae but have a patch of very long setae, and often several lines of setae radiating out from the tip of the stigmal vein. Elasminae is sort of like Entedoninae as well, but their wings are very narrow (plus they have a lot of other characters that set them apart - enlarged hind coxa, diamond markings on the hind tibia, scutellar lamella). Some keys rely upon whether the submarginal vein is smoothly joined to the parastigma or not, but I find that a hard character to recognize.