A tester screwdriver is a simple electrical device that you can use to see whether a circuit is carrying current. This fills a couple of useful needs--if you're getting ready to work on a circuit, you can see if power is flowing to it, and if you're trying to diagnose an electrical problem, you can use the tester screwdriver to see if power is indeed flowing to a non-operating device.
Remove any covers that may limit access to the circuit you wish to test. For example, if you are testing an electric outlet in your home, you'll need to remove the outlet cover's screw and then the cover itself to get access to the circuit. There's no one method to use here, since you could be working on a multitude of different electrical applications.
Identify a good point to touch the tester screwdriver to. It's important that the screwdriver tip touch a bare wire or bare metal connection to the wire, and not an insulated portion of the wire. For example, when testing an electric outlet, touching the tester screwdriver's tip to a screw that holds an incoming wire is a good plan.
Touch the tip of the tester screwdriver to the wire you're testing, being sure to hold the tester screwdriver's insulated handle. Look at the handle of the screwdriver. If the small neon light in the handle lights up, there is power going to the circuit. Otherwise the circuit is dead.