Nearly three years ago, I showed you an awesome $8 cable tester that quickly tells you if your USB-C cable is likely fast, slow, powerful, or weak. Sadly, that gadget got discontinued, and I’ve never found anything as intuitive or inexpensive since. But if you’ve got a Mac with Apple Silicon chips, you can simply download an even more impressive tester for free.

This free Mac app reveals the truth about your mystery USB-C cables. Called WhatCable, it works by reading the data your Mac already collects about attached USB devices, data that Apple doesn’t normally pass along to you. Just click a little widget that lives in the menu bar atop your Mac, and you can see every USB-C cable and device attached to your computer.

The e-marker is one source. WhatCable also reads from the Mac’s own hardware, the actual negotiated connection speed, Thunderbolt link speed, and live voltage and current at each port. The connected device tells us what it is, who made it, and what it supports. Put all three together, cable, device, and Mac, and WhatCable can tell you not just what everything claims to support, but what’s actually happening on the connection right now, and which part is the bottleneck if something isn’t performing as expected.

Want to see it in action? I took photos while testing some of my favorite cables this week. It’s not a perfect solution, as cables can lie about their capabilities, but WhatCable genuinely helped me find a bad cable along the way. The app offers more than just identifying slow or weak cables; it also helps with charging speeds and overall performance.

Morley isn’t the first to realize a MacBook could be a USB-C cable tester. USB Connection Information is a similar paid app that arrived a year ago. But Morley’s version is free, and he tells me it ‘will always stay free at its core,’ though you can pay £9.99 to get the Pro version that offers a real-time power monitor, diagnostics, and a terminal view. He’s also now built an even simpler version of the idea called WhatPort that simply monitors what each of your Mac’s USB-C ports is doing right now, including power, data, and video.