Alright I can see instantly that this is going to be the sort of post that few people will put their neck on the line and provide an answer to.
I will have a stab but I will add that I am by no means 100% confident of any foolproof way of dealing with this issue. Not wanting to tempt fate I have this situation but the complete opposite way around so it isn't really a big deal that my target is older than I assume. So on this note I would say that you should stick to your guns of assuming younger over older.
As for addressing the problem itself it's going to take a bit of trial and error. You're going to need to invest time into your observational skills. I wont go into detail here because I am by no means an expert but make quick observations of your target.

Who is she with? Her friends? How old do they look?

What is she wearing? How does it compare to what girls of your age group wear?

What is she buying/bought?
Those are some quick observations off the top of my head. Obviously don't stand there gawking and making it obvious you're looking at her, a quick glance at a distance should be sufficient, otherwise you're at risk of becoming that creepy guy.
If anybody could add to what I have said or scrap it and supply an alternative that would take a weight off my mind
