Battery Chargers - how do you know when they are no longer doing the job

[Rowdy]

  I have accumulated several actually many  chargers and tenders over the years - has anyone found a sure fire method of checking a charger and / or a tender - I'd would like to weed them out and get rid of the bad ones 

I know some won't register anything, some need to be attached to a battery with some juice left in the battery    If I hook them to a battery (That has some life) I should get reading from a volt meter when attached - right  ? Or am I just getting a reading volts from the battery  ???

Anyway - has anyone found a fool proff method to verify if a battery charger / Tender is till doing what it's suppose to do 

Cleaning out the garage of obsolite stuff 

[3W-lonerider]

sure fire method..hook up a volt meter to the battery and read the voltage. then with the meter still hooked up..plug in the charger..if the voltage increases the charger is putting out.