I typically use two strategies. The first one is exploration. There are couple of ways to do this. You can type in an application and see what shows up. For instance, type in “itunes”, then the right arrow key, and you see all of the things you can access in iTunes from Quicksilver. Another strategy for exploration is to open up its preferences window and see what’s there. There are dozens of actions and modules — what do they all do? Some of them will be useful to you and some won’t; it depends on what your personal workflow is.
The second strategy is when you find yourself doing something repetitively, to ask: how can I do this in Quicksilver? A lot of times, someone has already posted an answer somewhere (you’ll have to Google; this is a problem with not having the documentation in one place). For instance, in answer to your question about how to set events in iCal, I found this (first impression: it works but it’s a bit brittle).
These two approaches, which I found are the only valid ones to understand what QS can do, imply doing a lot of searching and fumbling around, at least compared to Launchbar or the terminal (both are much more documented).