This can be a little tricky, depending on what resources you have available.
First, you need to have an application that will give you a consistant and reliable throughput; NetIQ (www.netiq.com) has a free app called "Qcheck" that will report Throughput, latency, and packet drop on a small variety of connections (UDP, TPC, IPX ...). This will work for your QOS stream.
Next, you'll need some additional traffic-generation equipment ... it could be a couple of PCs with wireless NICs, Laptops, whatever you have. These machines will be used to raise the level of background traffic to a normally-impinging level.
SO, with all of that at the ready:
Turn off QOS for everything, create some sessions with the background traffic generators, Each machine doing a long file transfer will work. Try to drive the bandwidth utilization up to near max.
Then start the QCheck, and record what it reports as throughput, latency, and packet drop. Repeat this a couple times, then do the same thing reversing the rols of the transmitter and the receiver (do several runs and average for both trials).
Then, set up your QOS and repeat the above procedure. If you've done it right, the first trial (no QOS) will show the bandwidth pretty much divided by the number of machines participating in the test (four machines, ~ 2-3Mbps).
The second trial will show the QOS stream running with the amount or nearly the amount) of bandwidth appropriate to your config, and the non-QOS streams reduced by approximately the same amount.
Remember that the Wireless APs are performing as a traditional hub ....
Also remember that if you have multiple devices operating at a particular QOS level, they will all have precedence over non-QOS, but will still share the guarded bandwidth (through the wireless).