Heavies aside not really, I do confirm their weapon although the distinction is more auditory than visual

Pretty much always. Sometimes this is easier: Full Alpha Legion gear and Trophy Racks -> badass. Sometimes harder: Space Marine Devastator with Relic Heavy Bolter -> Might well die before I even get close enough to realise. The trim is a good indicator, but depending on the lighting it's not too easy to tell between the Red and Gold tiers. Chances are they're not a complete newbie if they've got trim though. Space Marine Halos are a pretty good visual indicator of badassery. The Eldar are probably the most difficult to differentiate, but that could be because I spend so little time fighting against them.

I do think the Marks of Chaos need to be made more obvious. Actually this is more for the benefit of me as a Sorcerer when I play Chaos, rather than as an opponent. There's really no simple way to tell your target has a Mark of Khorne until you try to heal them.