It feels like there is enough consumer frustration and confusion that this has been changing. A standardised mandatory marking in the spec would have been nice but it has been no problem for me to find cables with clearly-printed power and bandwidth ratings recently. History will consider this a minor teething problem.

not that it really solves your problem, but any compliant USB-C to USB-C cable is required to support at least 60W of power and USB 2.0 speeds.

Unfortunately, beyond that it gets pretty bad and spotty for a casual user. I make sure to only buy reputable cables that support 240W (this isn't an expensive feature: it just means the cable supports 20V, effectively), and I have only two cables that support the full Thunderbolt data rate. But those are effectively tethered to my Thunderbolt dock and I don't use them generically.