I picked up some cheapo usb cables from the dollar store that would recharge my tablet, but for some reason, wouldn't recharge a phone that used the same connections.
I would think you could immediately tell if the cable will or won't work to power the 70d depending on whether the screen pops up asking you if it's a storage or power connection when you turn on the 70d?
But yes, apparently not all cables actually work with all devices, apparently even if they appear to have the right connectors.