It can be hard, have just gone through this recently due to moving.

My main advice is to ignore rumour. If someone says a school is good or bad, ask why. Often their opinion is based on hearsay or out of date. As Artechim as says talk to current parents and go for a look.
You can get a good feel at times.

Schools can change though, so what is good/bad one year, can change the next, especially if there is a change of principal.