You can check for the power pins with a multimeter easily enough. You are correct that getting the data lines the wrong way around will do no harm, but I would check the others if you can. Don't assume anything about Apple's USB colour codings, I rewired one of their cables once, it had the conventional colours but they were not in the conventional order.
If all four wires are connected in a line, the power cables are typically on the ends, presumably to avoid shorting them by accident. Again, don't assume anything - you don't want to fry the on board USB controller. I have seen plenty of blown firewire controllers on plenty of Macs, USB seems harder to kill. Having said that, if you blow the USB you may be able to live without that extra port, but you may knock out the trackpad and internal keyboard too.
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