I'd smile back, give a little nod. Maybe just say a friendly 'Hi.'
I wouldn't necessarily go in for starting a conversation following that, though. Most of the time I wouldn't, I have to admit - generally one or the other out of us two is in a bit of a hurry. Failing that, I don't want to bother them without reason but if they looked like they needed a hand with something, or for that matter I really needed a hand with something - or if we needed to kill time, of course I'd go ask them!
If you meant in a romantic sense then... no, I'm not very good at clocking signals in general, never mind from girls I don't know. Some of my mates - female and male alike - would argue that's a downfall of mine, for me it just seems madcap to attempt form this crazy but somehow advised idea in my head of 'She just acted like a decent human being towards me, conclusion - she wants the D'. It can assuredly only lead to trouble, haha! I'm not that kind of dude, not very street-wise I suppose.
Extrapolating; but, if this is your way of trying to get attention from a crush or something along those lines then feel free to give it a shot a couple times, but if they're anything like as much of a numbskull as me they'd probably appreciate something refreshingly more direct when all's said and done! I think most, er, decent blokes will just assume you're being friendly, either that or that you're known to them.