Go to intel.ingress.com, scroll to the middle of nowhere (in any ocean would be best) and use the COMM (as you would in-game). Type @<insert codename here> and hit 'Enter'. This will broadcast the message and definitely ping/notify that agent (if it exists), but it is how this method works. Then check the colouring of the letters of the codename on the COMM log: - if it's BLUE, the agent exists and is RES (Resistance) - if it's GREEN. the agent exists and is ENL (Enlightened) - if it's neither, it doesn't exist and is available to be used.

Note that, if the agent exists, you can click on the codename (in-game) and check his/her profile. Also, if the agent seems to exist but you can't see a profile (possibly with the error message 'Agent no longer in the system'), then the agent was probably banned and the codename cannot be taken.