+1 on not judging a book by its cover. Personally, I have two very large tattoos, not prison tattoos, but they were done by a member of an outlawed motorcycle gang here in Australia. Granted they are not visible under a t-shirt, but soon I will have both arms covered right down to the wrist, as well as my shoulders and chest. I am a pretty friendly guy, with poor navigational skills in the city, and I often find myself asking for directions
If it were me (and this has happened often times, obviously I don't carry here though) if I see someone dodgey coming towards me, I engage them first with a slight upward tip of the head, and a quick "Alright mate?".
This works in three scenarios:
1: If they are looking to engage you in an un-friendly manner, it puts them on the back foot, as there is still distance between you and the interaction has already started.
2: If they just want directions they can go ahead and ask, as you have offered help already, and can feel good about helping someone out.
3: If they were just a bit of a space cadet and not really paying attention to where they were going, usually you'll get a brief "Yeah fine mate" back as they go along their way. Or they ignore you completely and keep on truckin'.
Take it or leave it, that's just how I deal with approaching ruffian looking people
-AE