When it is about new climbing partners you need to be very careful. This is not paranoia, simply necessity as you will be trusting them with your livelife. In any other situation, you wouldn't take that lightly either.
I found none of the options like asking them how often or since when they climb, looking at their gear, etc. conclusive, as many will overstate their abilities to not scare you away as potential climbing partner and anybody can and does buy great gear. So what to do:
Explain politely that you just want to observe her/him belay once first and of course offer the same for you. It usually is not that hard to find some additional person to climb a route while you two belay them. Then observe how the belaying goes. If you know the belay device your potential partner is using, this is extremely telling. You see clearly whether he/she is comfortable with it and whether he/she has dangerous habits. If you point out an error factually and nicely he/she might correct it promptly and you both gain something.
So do not climb with anyone you do not know or that is nownot vouched for by somebody you know to be absolutely trustworthy unless you have seen them climb. And I would recommend not to count some "certificate" from a course, that may be a one hour of instruction, which is not sufficient to make a proficient belayer.