A bit of context: I have a 6-port/60W USB power hub which seems good to power a few raspberry pis. I've tried it with a device in each port (raspis and a banana pi) and all seemed right - pretty stable voltage at 5.13 volts, I'm not sure what else to measure.
However, watching a movie in a raspi2 with openelec and nothing else connected to the hub I got the power/temperature warning icon, and the temperature was normal.
I've read that a common reason for this icon maybe a low quality cable. Thus my question: is there an objective way to measure quality of an USB cable?