the exact same way as you or i would obvioulsly.

you see a line, then try to make the line wider.. does it get wider with force? yeah, then its cracked.
maybe push your finger into the "line" and see if there is some kind of movement there.

I sometimes wonder if the whole of humanity suddenly went completely nuts.

If you were to present this "problem" to people living in the 50ies, in the US. I'm fairly certain they would have solved it in like 1 minute. just like me. carbon is no different to other frame materials when searching for suspected cracks imo.

But people are afraid of carbon. they don't want to touch the holy carbon, made by the taiwan mass production buddhas.

I'd say if anyone human can make it then anyone human can inspect and determine if its shot. easy as that. and it actually is easy as that.