Have a crush on a girl from Tesco

Pretty girls in retail get asked out a lot I guess. They probably begin to see it as an occupational hazard plus you've know idea if she's single. I would at least try to flirt a bit and introduce yourself to judge interest before asking her out.

I only know her name, been too shy to ask her anything else. She doesn't wear a ring, she always works at the express checkout (5 items or less, so she's fairly "distant" from the other shop workers etc).

I dunno what to do, to I dunno: well, build up to asking her out

I think this is beautiful but if you're going to ask her out, do it not while she's on the job. As far as asking you've nothing atall to lose. She sees 100s of people everyday and surely has either been flirted with/flirted back harmlessly or has learnt to separate work from that and might just see any thing you say to her as friendliness. I recognise all the people at checkout and they recognise me, and we chat and I chat with the girls and back and then take my bags and go. If you actually flirt with her while she rings up your apples it'll be embarrassing.

Unless you hand her your bank card with your phone number on a piece of paper. Smile at her and maybe a cheeky wink and say see you later. See what happens.

Not trying to be awkward seriously

But you say:

"do it not while she's on the job."

But where else will I ask her....? Can you maybe clarify/explain that for me?

Its not beautiful, just a troll hoping for someone to maybe come brighten the bridge

I already wrote you can slip her your digits while you slip her the bank card if you have one
And if you don't wanna do that then chat her up like a stray drunk while she cordially ignores you, slams the cash register closed, and hands you your change and yells "next!"
Or of course wait for her to finish her shift...which is creepy.
So I'd go for slipping her the digits and small talk for beginners.