Is there any chance to distinguish pepper variety from just seeing their leaf? It would be interesting if one has the knowledge, and would share it to other.
Nope, not in terms of species.
You can generally tell a chinense due to the big ol' leaves, and some other, more varied genera you would also be able to distinguish. But that's it until flowers & pods appear.
It's easier to tell the pubescens apart from everything else because they tend to be hairier...As far as between similar species that would be quite a challenge. There are some unique traits to certain plants that could help, though. For instance, I got a mix of seeds from a free thread in the marketplace and one of them that popped up was a Varigated Tricolor (according to AjiJoe, the originator of the mix). Google some images of that and you'll see why it is easier to tell.
Also, if you have some purple types they tend to be easier to pick out, but if you had multiple types of purple plants I imagine that would be close to impossible.
I can distinguish a fatali and peruvian white habanero just by sight every year without exception...heck, even some annuums like the charleston hot are instantly recognizable at the second or third set of true leaves. Some varieties of the same species, particularly those that have been stabilized for a long time, just show very distinct characteristics that the trained eye can pick out. Obviously it's a lot harder to tell with varieties you are only growing for the first time, because you just aren't that familiar with what they typically look like. I have also been able to spot unintentional crosses way before they produced fruit just by comparing the way the seedlings look. YMMV
It's difficult to tell within the same species. I mixed up some fataliis and devil tongues. I couldn't tell the difference so I tossed them. However, I can tell the difference between some of my peppers within the same species. A white hab looks way different than a naga morich for example.