The flow test I took involved urinating in a receptacle that measured the
milliters of urine pissed out per second.  They made a small printout.
There is also a type of flow test where they use a cystoscope to pump your
bladder full of water then have you urinate it out.  This one is a bit
painful. I did both of the tests.

You pee into a device that times how long you are urinating and how much you
pass. It also works out the peak flow and slowest rate and works out the
average. It prints out a ticket with the data on it. They then do an
ultrasound scan to measure the residue left in your bladder and then you pee
again before another scan.
Derek.