How can I quickly tell if the receiver on my handset is broken?

With the phone laying face up, drop a small metallic object like a screw or a paperclip on the receiver of the handset. There's a little magnet in the phone that should keep the metallic object in place. See the illustration below:

If the object falls then the receiver has been damaged. You can stop at this point and RMA the handset.

If the object stays in place, then the receiver is most likely working, but there's a small chance that it may be out of alignment.

To verify that the magnet is out of alignment, you would need to replace the handset with a known working handset and see if the issue persists. 

If the known working handset works, then you will know that there is an issue with the handset.

Otherwise if the known working handset does not work, then there is likely nothing wrong with the phone.