I know about cmd-double-clickand use it all the time, it's great. But is there an easy way to make Xcode just tell me the type of character in a pop-up or the like, without looking up from my current position in the code?
option + click to find the standard documentation there.
For custom characters not sure. You can click command+ option+ to open the definition in the helper editor, in Xcode 4 +.