Hi Franko,
I've had a look at the one at VW Alley
http://www.vwalley.com/
and at CIP1
http://store.cip1.com/Merchant2/merchant.mv?Screen=PROD&Product_Code=ACC-C10-7095&Category_Code=*MS-01
which I assume are typical.
By the looks of it, you bolt this onto your (in-car) engine, pull the crank pulley towards the back of the car as far as possible, wind the screw in till it touches the pulley, lock it with the lock nut, push the crank to maximum forward and then measure the gap between the screw and the pulley using feeler gauges. I made these instructions up looking at the "tool"; if they turn out to be better than the packaged ones, I want copyright!
You could make such a tool or have one made very easily; the $64 question is does it work? Well, it's not going to be very accurate; it relies on the accuracy of a normal. no-machine-grade bolt's threads and they're not very fine threads either; it has got six ways to flex, and you're trying to measure something that should be 3 to 5 thou (0.003 - 0.005 "), you're only going to get a rather rough answer. This may not matter for an in-car measurement; after all, you probably only want to know whether the engine is OK or a junker. Once you've measured the end play the only decisions are simple binary ones; buy/no buy, recondition/don't recondition etc.
You could do the same job much more accurately using a dial indicator, which is the conventional way. Depending why you're measuring the accuracy difference may or may not count.
The tool at CIP1 looks the better made of the two; at USD 12.95, say AUD 25 landed here, I bet you could get your local machine shop to measure the end float with a dial indicator for less...
hth
