The answer is that it is almost impossible to separate Lasioglossum (Chilalictus) species from photographs. L. lanarium is, however, one of the largest and most common species in the subgenus and pictures of males often show the end of the antenna turned slightly outwards and the apex is flattened not rounded. I can't guarantee that males of other species never look like this, but I have not seen it, so as long as the size and general hairiness look right, I use it as an indication that the bee is probably L. lanarium.