The questions that tell you how a thing behaves on a busy night, and what to settle about your own site before you talk to anyone selling.
Every quote you are sent describes a thing working. The decision you are actually making is about the hour when it does not: a counter with a queue in front of it, one person behind it, and nobody free to come and look. That is the condition worth judging anything against, and it is the one condition no demonstration ever takes place in. So before you compare two options, write down the three or four moments in your service that genuinely hurt — the counter that goes quiet, the order nobody can find, the item two bars both think they sell — and treat the purchase as an answer to those. A feature that touches none of them is not a reason to buy.
This holds whether you are looking at a till, a printer, a tablet, a card reader or the software joining them. The questions below are deliberately not about what a thing does. They are about what happens around it: who notices a failure, how long it takes anyone to find out, and what you still own at the end.
Half of the real questions are yours, and no supplier can answer them for you. Walk the site with a plan of the bars and settle them before you spend anything, because the answers change what you need to buy and quite often mean you need less of it than you were about to order.
The worst moment to discover you bought the wrong number of something is the morning of an event, and the second worst is the week before, when nobody is answering the phone. Leave room for one full rehearsal on site: the real menu loaded, every counter switched on, a few orders put through each of them, the paper taken out of one on purpose so you can see what you find out and how. Then write down what you bought, where it lives and what it is plugged into, counter by counter. Most trouble on an event day is not a fault but a missing fact — which socket, which roll, whose login, which counter the spare box belongs to. That list costs nothing, and unlike the knowledge in one person's head, it is still there when somebody else is doing the setting up.