What do you mean by "live"?
Why don't you just assume that they are all "live", since unless you
have taken the steps to shut off a particular circuit (and do it
correctly), pretty much any wire up there could go "live" at any moment
(for whatever definition of "live" you want to give).
I don't see what you are trying to do here. If you happened to find one
wire that had current passing through it at any particular moment, that
still doesn't tell you if it would be a good one to tap into (maybe its
on a light switch, or a dedicated appliance circuit, or 240V, or a
timer, or a thermostat, or who knows what else). And even if it didn't
have current passing through it at the moment (and some line testers
won't tell you unless there is current), it might still be at 120V or
240V, and you don't want to touch that either.
The only "dead" circuit in your house should be the ones that you
yourself shut off (and made sure no one will turn back on).