A successful way to tell if you have a good babysitter or not is if they follow your expectations. BUT, and this is significant, you have to tell them precisely what your expectations are, AND what you expect of the children too.
In today's world we can't expect that common sense is common, that babysitters will not be on their devices while they are babysitting, that they won't just plop the kids down in front of some screen, that they will clean up after dinner, etc. I know, you'd think these things are a given but not anymore.
So in order to give you and your babysitter a fighting chance for success on all fronts here are a few ideas:
Set precise rules in front of the children about screen time. Screen time is a huge issue these days for both children and babysitters. You can't expect babysitters to know what your rules are in regards to this. Every family is different in regards to screen time, and what a babysitter is allowed. My agency, Babysitters In a Pinch; when interviewing, informs babysitters that they are not to be on their phones etc until the children are in bed.
Know your expectations ahead of time. It is important to know what you expect from your babysitter before they arrive. This shouldn't be hard because expectations are usually the same from time to time with very little variance: Will the kids need baths, put dinner dishes away and wipe down the counter tops, have the kids in bed at a certain time, fold laundry, etc.
Write down your expectations along with emergency numbers. Sometimes in the chaos of handing off children to the babysitter expectations that are verbalized can get forgotten. Having a written document to refer to when it comes to bedtime routines, discipline techniques, how many stories for bedtime is very helpful for a babysitter.
The house should be the same way if not better when you get home (as far as cleanliness goes). Duh, right? Not necessarily. In my mind and probably yours you shouldn't even have to say that or communicate it at all, but if you don't you can't be mad if it isn't. Trust me, I think that it's stupid to have to state the obvious but anymore you can never assume anything.
Make sure the kids know what you expect from them while you're away in front of the babysitter. We all know that the minute you walk out the door the kids start telling the babysitter all sorts of things that you allow them to do that you don't allow them to do. I had children telling me stuff like they could have soda (Coke) right before bedtime, and that mom and dad allowed candy whenever they wanted. Of course, I'm savvy and I would just say; "Well tonight no soda or candy. If mom and dad think that's ok that's fine but I'm thinking that isn't correct. I'll check with them for the next time." This usually takes the wind out of their sails. Recently I had a babysitter out that the kids said mom and dad said it was ok for them to draw on their bedroom doors. Puh-lease!
Be realistic. Most babysitters can't be as organized, and as fantastic at multi-tasking as you are. You can make a healthy 3 course meal, help with homework, keep toddler out of trouble, occupy your pre-schooler, and clean-up the kitchen without even thinking about it. Babysitters aren't in a routine with your family like you are, they aren't sure what makes the rhythm of your family tick. So expecting them to step in and do the same things you do without hesitation isn't fair to them or your kids. So when you think about your expectations be realistic. Having the kids in bed on time, having the kitchen orderly upon your return, toys picked up and put away; these are realistic expectations.
All-in-all you want to have a successful experience with your babysitter so that you can have them return, and become a steady person in your family's life so if you set your expectations upfront you'll have a more successful experience. If your expectations aren't met you can choose not to have them return or you can give them a second chance but let them know that their performance wasn't acceptable, and that you expect more the next time around.