It's defo a British word...when *we* (americans) use it it's because we're expressing our anglophilia and repeating something we heard over and over again in song lyrics by mod/skin bands.

like the 4Skins song "wonderful world"

"Old bill knicks you for suss" (prolly referring to how bobbies would have skins remove their laces at football games)

I dont think theres a real US equivalent of the word, so we just co-opt yours

'Sus' almost certainly come from the so-called 'sus law' whereby under the Vagrancy Act of 1824 a police constable could stop, question and search anyone"loitering in a public place" simply on the suspicion that they might be intending to commit a criminal act.

The sus law was notoriously used in the 1970s to intimidate 'counter-cultural' elements (ie people who were not white or middle class).  By inversion 'sus' became part of counter cultural slang meaning to check something out or gain a full understanding of it.  In the late 1970s a sussed out person would really have got it all together (as it were).

I always liked "butchers" for look.  I remember hanging with a couple of English guys from London in a what will remain unnamed foreign country years ago, on the piss, heavily of course, in a bar with many delights.  Many a butcher there.

I think it sounds, ridiculous to say "sussed" for an American resident, maybe not suss, which could be ok as  "don't you suss me Spanky", as it's very Alfalfa.