There are two ways to tell if an egg is fertile. One, put it on an incubator and see if it develops, or two, crack it open and look for the bullseye.
I would highly recommend against putting new chicks in with two week olds. The two week olds are likely to kill the younger chicks. If you have more than a few days age difference, wait until the youngest are about six weeks old before putting them together.
Do not release coturnix quail outside. They are domesticated and don't have much in the way of survival instincts. They also don't come "home". Everything likes to eat quail, so that lowers their chances of survival even more. It's also illegal in most places.
If you want to put them outside, make sure that you have predator proof pens for them and enough space, hiding places, and shelter for them.