How do I measure the voltage coming out of a USB port?
The standard pin numbers and colours in a USB cable are shown below.

Pin Number   Cable Colour   Function
     1          Red         VBUS (5 volts)
     2          White       D-
     3          Green       D+
     4          Black       Ground


If there is no device connected to the USB port then the maximum current that the port can support may be quite low.  Once a device has enumerated itself on the port then it can negotiate for up to 500mA from the host controller.

To measure the voltage then you either touch the meter probes onto the stripped ends of the wires or, if your cable end is a large connector, then touch the probes (without shorting them together) onto pins 1 and 4 of the connector.  The power pins on the USB connector are easy to identify as they are longer than the data pins.  This is so that they mate first and ensures that the power is connected before the data.