Depends, but I ask to see the print out/screen. I have a battery tester now, but ignore the good and look at the numbers.
EDIT:
It is worth saying, that once you've found the correct numbers from the battery, then plumbed them in, then done the test it's not easy.
You have to consider is the battery connected, temperature and a number of other factors, plus what the car is likely to need to crank.
My last battery was showing good, but only 67% capacity, which might be ok for certain jobs, but certainly not ok for cranking a diesel in winter. I got the first hint when it took a bit too long to crank the engine on a cooler day, but was fine on a warmer morning, then bad again as it cooled even though I'd charged it.
Sure enough new battery, which I charged before swapping started on the dot and stopped a couple of odd behaviors that I'd seen once or twice. When sensors use 12V as a reference voltage and it isn't quite 12V that they're getting things can get really odd, really quick.