How do you know when a wild bird egg is going to hatch?

i found this cute little bird egg on the ground. above it was a torn nest that looked like a squierrrl got into it. and also, a crushed egg was next to it. had it for a while now, but i dont know when its going to hatch....... what are some signs for the egg about to hatch? ive been incubating it, but dont know how long to go on. any ideas?!?!?!?!?!!?!?!!