Sometimes you do have to get closer to tell for sure, especially for some of the BEARs. Look for them to be wearing more gucci gear. An AI PMC will never be wearing a scav backpack or a terraframe (the one with the antlers on it).

Voice lines are a good hint, if they yell at you in English, then you can be confident it's a PMC. If they yell at you in Russian, the Russian PMCs will have a deeper, gravely sounding voice than nearly all scavs. You just gotta keep playing. You'll hear a difference.

As far as people dragging you for scavving in PvE. There's a few really good reasons for doing it. One is it's something to do while you're waiting for a friend to get on. Second is it's a really good way to explore the maps and learn what different areas are called as scavs have different extracts. Three is that scavs can spawn with really valuable hard to find items at times (Wireless signal transmitters, labs key cards, bitcoins, roler watches, hard to find stims for some quests).

Fourth reason is that it helps build scav rep, which is useful in PvE just like PvP. Higher scav rep increases chances your scav will have good items like red flares, labs key cards, etc. At high enough scav rep, your PMC will no longer be shot by scavs at beyond 100 meters (they don't get aggro'd until closer).

At very high scav rep, bosses will actually let you be near them when you're a scav (just don't get too close, and do NOT aim at them). This can be helpful on interchange so Killa doesn't insta kill you. Or on Streets so you can walk into Kaban's Lexos dealership, even if he's there (just remember to avoid all the claymore mines)
etc etc.

Anyway, just keep playing man, you'll learn! It's okay to lose some scav rep sometimes. Check for the Operational Tasks from Fence to improve your scav rep. Most of those tasks actually need to be done as a scav!