You must get opinions from many different sources. Teachers or ex teachers, friends,(you can tell alot by the type of friends) neighbours, aquaintances, classmates, job mates, etc. A definate picture will emerge.
You are asking what to look out for when dating. It comes before dating. You or your parents have to make numerous enquiries to find out if the girl has yiras Shomayim and midos toivos. Then if you have those 2 also, you will have a successful marriage. You must give and give. Have no expectations.