Your Subaru+iPhone combo already tests that capability, does it not?

Just purge the known-bad cables already. :) (Gather them up and scrap them if the landfill seems like a harsh destination.)

In a situation of surplus, there's no reason I can think of where keeping charging-only cables around has merit.

Later on, if you need a cable that very specifically can do USB 3 speeds, then: Use a flashlight and look inside of the USB A connector. The ones that only have 1 row of 4 contacts will never be able to do USB 3, while the ones with 2 rows contacts will probably work fine.