How do you catch someone who’s lying about not knowing english?

Why do you care ? Maybe she doesnt want to be on the cash register dude, and maybe she reads english better than she understands it spoken or can speak it herself. Just let her do her thing. It's not your problem if you're not a manager, obviously if the managers do nothing about it they dont care. Maybe she's talked to the managers and they understand.
Keep your nose out of it. You're like the worst type of co-worker.

Some people just read languages better than they speak them. I'm definitely like this, personally. It doesn't help when people interrupt you constantly and stare at you as you try to talk either. It obviously depends on the individual circumstance- who, what, where, why, when, etc etc...

My personal suggestion would be to just communicate with her. Some ideas:

-Learn her language
__-this could be written or verbal. If she likes reading just write her a note.
-Use Google translate or something like that. Just learn what language she speaks and give it a shot.

Shocking and enlightening, I know.

Just do your job doofus, sheesh

If she's fluent in the language she claims to be only able to speak, there's no way to prove it. You'll just have to suck it up until she slips up one day.

However, if she's claiming to not know English but only knows the basics of her "home" language you can learn a phrase and try saying it to her.

“When shes on burgers she reads all the orders correctly”

If she can read English, she speaks it. Reading it is actually harder in fact. People can verbalise plenty of words they can’t read or spell.

Bitch is lying.

TL;DR: taking a quick skim read of the comments, it looks kinda contentious.

To answer your question, you can tell by checking for involuntary reactions to what you're saying. Someone who doesn't understand will just look at you with one or another shade of 'lost' on their face pretty much no matter what you say. But if their face shows an expression anger, fear, relief, disgust, agreement, disagreement or some other emotion immediately after they've heard you say something, or if they laugh at something you say, then they have understood. Or maybe only partially understood and got 'the wrong end of the stick', if the reaction isn't what you'd expected...

Idk, id just leave it alone really. Its just unnecessary drama. That sounds fuckin annoying though

If someone lies about their limited English then they're an idiot.

tell her that her pussy stinks and see if she responds

Knowing English and being professional in a language are two different thing. I know english. But not a professional. Working in a cashier, a caller center etc where talking is involved, need good communication skills which doesn't come from just knowing a language. To deal with obnoxious customers limited vocabulary is not enough. A person need a good, healthy vocabulary with a good come back skills that doesn't sound offensive

some people have just enough understanding of a language to read and write but communicating/socializing is an entirley new beast. Theres certain slangs, innuendos, accents and varying speeds. It's really difficult for a non english speaker to understand everything. I can read and speak some chinese, I could probably read burger names and stuff but communicating would be hell. They would speak fast as shit and the dialects and other new words, questions, it's a lot.

If she doesnt want to do it, thats a her problem not a you problem