How do you know ELT's batteries used more than 50% of their capacity?
There are two ways:


First, when you test the ELT you should make a note in the aircraft logbook of when it was tested and the results of that test. You can also note how long the transmitter was on during the test. You can then look at all the logbook entries to verify that the cumulative time is less than one hour.
Some ELT's have a self-test feature that presents error codes via an LED or audible beep when they are powered on. For example, here is the self-test page from the Artex 345:




The ELT will beep/flash 7 times when the battery operating time is &gt;1 hour.

As far as the useful life of the battery goes, that is determined during the manufacturer testing for the ELT and will be marked on the ELT as an expiration date. Here is an example:


Source: Aerotechservices.com