How can I tell that a collar is not reporting? 

A non reporting collar will have a yellow icon in the app. 

There are many reasons why a collar is not reporting. 

The most common reason is that the collar is in an area without mobile connectivity. In this case, the fence, sounds pulses still work, but the collar is unable to communicate with the app and you will not be informed about the animals location, pulses or escapes until the collar is reporting again. 

To know for sure if this is the cause of a non-reporting collar you can either move the collar to an area with mobile connectivity or go physically close enough to the collar to connect with the collar over bluetooth.