It should be a bit of a clue when you get back to her place and find the Justin Bieber posters are almost obscuring the Spongebob wallpaper in her bedroom

Yes.

Just don't pick up and shag a girl before getting to know her, at least enough to know whether she's over the legal age of consent. It's really not that difficult. Just keep your penis in your pants for a while.

As long as you know the lass is 16 years old then you have nothing to worry about.

Once again, we get back to the tricky part. How do you find out how old she is other than checking some kind of official ID?
How do you tell the difference between a girl who is 15 years 11 months and a girl who is 16 years and 1 month old?
As mentioned by others, a confident girl can often make herself look older, and girls (and lads) have different interests at different ages and mature at different rates, so it's not as simple as just getting to know her a bit.
Not that it's really an issue for me, 16 year old lasses weren't interested in my ugly mug when I was 16, never mind now.

I see Jim Davidson got rearrested again this week on sex allegations.
How about throwing crimes against comedy in with it?