About all quarantine will do the way most people do it is tell you if the newcomers are already sick with an obvious disease and the person keeping them did not recognize it or that they have been exposed to something recently. You have to look for symptoms, whether those are trouble breathing, coughing, mucous or some discharge, really strange poop, or they are scrunched up, lethargic and not eating or drinking. Some symptom that something is not right. It is also a good time to treat for worms and check them for mites and lice.
It is not a guarantee at all. It is still possible that they carrying a disease but they have developed an immunity to it. No matter how long you quarantine them, they won't show symptoms. It is also possible youir chickens are carrying something but have an immunity to it and will give it to the newcomers. There is one side bar to this. Occasionally a chicken has a disease but will not show any symptoms. But since relocation and quarantine can be stressful, sometimes the stress will weaken the chicken enough so the symptoms show up.
Bottom line is that it does not guarantee anything but it is another one of those things that improves your odds of success. Quarantine will catch some things. But part of it working is that you recognize a sick chicken when you see it.