If you have the bq27x00_battery module loaded, it is included in kernel-power but not the stock kernel and I think it needs to be manually loaded (or added to /etc/modules) on older versions of kernel-power, but it loads automatically on the latest version. Then when you type the command "lshal | grep voltage.current" you get two results returned, one will be from the bq27x00_battery module (should be the second result, but I'm not sure it always will be) and this one will update when charging to give you the voltage in millivolts.

Is there any reason why you think it won't have the correct voltage? It will be regulated by the charging circuit inside the N900 and not directly affected by the voltage supplied through the USB port. I would expect (but haven't and am not willing to test) that if the USB voltage is outside a certain range it just won't accept it.