With all the worries over bad USB-C cables and vendors that play fast and loose with the power spec, here <https://arstechnica.co.uk/gadgets/2017/01/usb-c-power-meter/> is a measurement gadget that can help to some extent. But as the article says: If you really want to test a new charger or cable before plugging it into an expensive device, your best bet is some DIYish electronics involving a breakout board and a multimeter. But there doesn't seem to be a plug-and-play version of that yet.