If you hold to the light and see glass between the crack edge and surface, then your phone screen is cracked probably, if you just see the crack on the surface (or feel it), its probably just the screen protector. (this is similar method to detecting one-way mirrors - glass in between reflective surface and outside surface is normal mirror, reflective surface on outside of glass, might be one-way and someone is watching you.)
The glass screen protectors can usually be taken off very slowly and reapplied. They protect more direct surface scratches and impacts (drops). They will not be as useful against stress cracks (Bending the phone past point of screen tolerance, like sitting on it in back pocket or tight jeans pocket), which would be the case probably if the protector is intact and hte screen is broken.
