Is there a cheap and easy means to evaluate iPhone chargers and cables?
The only thing I have found that consistently works is a USB-C power meter, such as this one from Satechi:
https://satechi.net/products/usb-c-power-meter?variant=31043692041
I have tested many combinations of chargers, adapters, etc. and this will give the correct information. I found that some combinations of chargers and adapters work differently with for example a MacBook Pro and an iPad Pro - so getting the reading off a MacBook Pro might not reflect what actually happens with an iPad for example. This has happened to me even though the iPad Pro on paper should support that same voltage/ampere - and probably does - but for some reason having an adapter in between the charger and the iPad Pro makes for a different scenario.