What is best way to determine if crack in root canal and crowned tooth has extended past the gumline? crown is still sealed with temporary cement and should come off?

Visualization is probably best at this point. After the crown is removed your dentist will have better access.

Have a root cana specialist, an Endodontist, remove the crow and examine the tooth under an Endodontics microscope, not a piece of equipment that most General Dentists have in their offices.

The extent of the fracture should have been visualized at the time of the root canal. If you are still having pain when you bite on things the occlusion should be reevaluated and/or an endodontic referral should be obtained to visualize the tooth with a 3D scan. A normal xray does not always pick up a fracture.