IF you are very used to growing particular strains, you can sometimes identify those plants on sight, IF you know what the possible choices are. Like if someone had 5 strains in a tent and I was familiar with all, and I knew there would be no other odd strains in there, I could probably ID them accurately.
But there are thousands of named strains and millions of crosses. And the gene pool for Cannabis may not be very deep. A lot of plants look the same. It would be a shot in the dark for me if some random plant was presented to me.
Yeah it's almost impossible to tell a specific strain just by looking at it. Most growers can identify one from another if they've grown it for long enough and there's enough defining characteristics (usually in flower) to tell them apart. The ONLY strain I can personally identify any time any where, flower or veg, is Forbidden Fruit. Only because I have exclusively grown that strain for a long time now. Even then, my key indicator is not look but the smell.. Forbidden Fruit as a very, very unique smell even in veg.