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To verify whether a circuit is dead, a two-pole voltage tester to DIN EN 61243-3 is the right tool. It measures between two contact points and shows via LEDs, a display or a moving-coil indicator which voltage is present. Many models also test continuity, phase and phase rotation and have a load switch that lets you detect capacitively coupled phantom voltages.

Electricians follow the well-known five safety rules: disconnect, secure against reconnection, verify that the circuit is dead, earth and short-circuit, and cover adjacent live parts. The two-pole tester is the tool for the third rule. However, work on the fixed electrical installation belongs in the hands of a qualified electrician.

Non-contact voltage testers detect the electric field of live cables and beep or light up as soon as you get close. They are handy as a quick preliminary check, for tracking down broken wires in fairy lights or for checking whether a cable is live. They are not suitable for safely verifying that a circuit is dead, because shielding, insulation or battery level can distort the result.

For the same reason, the classic single-pole phase tester screwdriver is not a suitable measuring tool – at best it's a rough indicator.

There are also specialist devices for typical DIY questions:

With two-pole testers, look for the DIN EN 61243-3 standard marking, the measuring range for AC and DC voltage, an adequate protection rating for outdoor use and well-protected probe tips. A load switch and a display that works without a battery increase safety.

Check every voltage tester on a known voltage source before and after measuring to make sure it works. That's the only way to be sure that a zero reading really means the circuit is dead and isn't due to a faulty device or a flat battery.