Test for disarm

Just be careful even with the fused test line method, because using a 1A fuse might still be trouble to the electronic components, even though it'll protect the wiring. If it's a logic signal you're trying to drive rather than a relay, you could try putting something like a 1k resistor in line, which would limit the current to a safer level while being able to drive a high impedance input. If the circuit you're testing is driving a relay coil, it'd need more current, so you could either use a low current fuse or smaller resistance in line to safeguard against a misprobe. I'd always try to start with a DMM voltage probe to first see if it makes sense; e.g. if it's reading ground already, it obviously can't be a ground triggered input. Checking the open circuit voltage to see if it's 5V or 12V also could give you some indication if you have the right wire. If you're fairly sure you have the correct wire, you can skip the preliminary checks and try actuating it, but better to be safe if there's any doubt, since it could get expensive otherwise.