> I find it hard to believe there's anti-sub tech that can tell if a given vessel is being operated by cartel members or the Russian navy.

Without betraying any secrets, the army can know a lot (I mean a lot) about an enemy submarine by its acoustic signature alone. Picking apart a drug smuggling sub from a military sub is child play provided you can actually hear the military sub which is easier said than done.

> they can't tell without boarding the sub

The USA can't board an enemy sub unless it surfaces. That's not what an ASW operation is.