I recently bought a bunch of outlet testers by Klein Tools. They are $20 at Home Depot.

These orange ones replaced the old yellow ones that I’ve had for many decades. The difference is that the new ones have a text readout in addition to the three-light pattern that shows missing ground, reversed hot and neutral, etc.

Why is this important?

I carry several of these in gig bags and won’t plug any audio equipment into an outlet that is mis-wired. An outlet incorrectly wired can cause damage to your equipment or injury to you. It can also be the cause of an annoying 60 Hz hum in your recordings.

45 years ago, I ran an audio retail store. We had high-end equipment on display on a wall with shelves. The units kept malfunctioning or going dead. We discovered the outlets had been mis-wired with hot and neutral reversed and a floating ground. I’m lucky someone wasn’t injured.

I too carry one. Useful, not only on your own electrical, but on what it is been supplied to you.

One of the extension cable given to me had the positive and negative reversed.

Another time, I had checked an outlet, which was ok for a short time. Then I felt a little tingling sensation while operating the mixer, I quickly re checked the socket and the earth was missing. Something to do with a loose wire. That got fixed before I reconnected the equipment.

It's a good tool to have around and for less than £10 on Amazon, it's a no-brainer.

I still use the older, yellow ones, work great. I ended up buying this years ago after 2 guitarists in the band I was in at the time were plugged into opposite wired electrical outlets. The touched their guitars together during a performance and as their strings touched sparks flew everywhere! Melted strings and fuses blown. Luckily, equipment was not damaged.