Probing nets can be done using geAddNetProbe() or with one of the geMakeProbeWithColor type functions.Start cdsFinder from the UNIX command line and look for ge.*Probe - and then search for the function you want in cdnshelp (if using IC61X) or cdsdoc (if using IC5141).
DRD is quite a complex piece of software, so you're not going to be able to write that with a few lines of SKILL code! Hardly a good place to start learning how to write SKILL. I wouldn't be able to give you the code, or even the algorithm outlines as it is proprietary, and anyway it would be way too long and complex...