1) You need to indentify the fan then look up it's voltage ratings/power.
2) the max output from the usb port is 5 V @ less than 0.5 Amps.
.. You can not do anything about the output power for the USB port. The 5 volts is fixted and if you load it so that you try to draw to much Power (current), the port will (a) limit the current, (B) shut the port down, or (C) you end up damaging the port.
You can use a digital voltmeter and measure the voltage to the fan with the control turned to max. If it is at 5 Volts then the ONLY thing you could do is replace the fan with one that will provide a higher CMF @ 5 V. WITHOUT exceeding the current limitations of the USB port.
DVMs are cheap (about $15 ->$20 @ walmart) and very simple to use, Just Google "How to use a voltmeter"