How can I tell if someone is sexually and/or romantically attracted to me

Make a move that indicates interest in a manner that is socially acceptable and effective. This seems to be both an suitable application of empiricism and an adaptive interpersonal strategy in a non hostile social environment.

if a combination of living in a country with somewhat lenient proxemic norms and having the “nice guy privilege” means that all the obvious ways to tell yield lots of false positives?

Again, using a heuristic that returns false positives (or, in objective terms, using strategy that includes active investigation even with non extreme estimated probability of interest) is an effective interpersonal strategy in a healthy social environment. If the environment is such that making social overtures when it so happens that there is not interest comes with a particularly high cost then it is probably best to find a better tribe. (Or to compartmentalize the forms of social interaction that you do in your various locations---a strategy otherwise known by such catchy yet crude morals as "Don't shit where you eat!")

People smile at me, compliment me, touch me, buy me drinks and give me lifts in their cars all the time, even when they're in a committed monogamous relationship with someone else and even in front of their boyfriends/husbands.

Your friends, acquaintances and/or people you casual interact with sound great! Good for you (sincerely).