Well I built myself a testing rig comprising of an old usb cable and old multimeter which was cheaper to replace than buy new leads and for the record cheaper to replace than replace the 9v battery within.
So I got the leads and cut them down so the usb lead had just a bare red and bare black wire to solder to the now bear ended (and no pointer pin thing on the end) multimeter cables.
Once soldered it looked like this (electrical tape used to tidy):
Now I can switch the multimeter to voltage and plug it into a usb powerbank or any socket on a usb hub and measure the voltage coming through.
I know the positive and negative are the wrong way round but just for a quick picture it will do.