I've done a little more research on the whole A/F / ego thing, and come up with some useful info. It seems that for a non-permanent solution, you can just hook up a DMM (must be digital with high impedance, or you will screw around with the sensors operation), and watch the voltage output.
As in the following links, it should be 0-1V (0 = ultra lean, 1= ultra rich) - but the response of the sensor does not seem to be linear. I thought this was especially interesting, since depending on the design of the more expensive jaycar kit, it could be less than totally accurate . . .
If nothing else, it gives some good ideas on how to test them, and check that everything is happy (something I'm going to have a bash on hopefully tomorrow...)
Follwing is also an interesting graph from one of the pages. I'm not sure how closely all ego's respond, but from the various info I can find, it seems that they are all fairly similar. However, if anyone has any info specific to the hego that we get in Falcons it would be great ! :)