It's just one of those things — although Adobe's Help on GREP mentions 'line' several times, ID's version of GREP does not really support lines, and where Adobe sez 'line' you are supposed to mentally translate it to 'paragraph' instead.
This is one of the things that you cannot do in the interface but can with a little scripting: if you search for en-dashes, a script can tell you which line it is on, and even on which line the next character is. My little script does just that: it searches for “en-dash space any character”, and then checks if the line for the first character (the en-dash) is the same as the 'any character'. If not, it applies a character style “Invisible”.
The style “Invisible” is a simple style, with only the fill color set to None. But: the very first thing the script does is … setting the fill color to black, and then throwing it away. That's because in a script you cannot delete a style without “preserving formatting” (at least, I cannot find a way to do so). Throwing away the style means everything in your text boxes gets reset to showing all en-dashes; then the script checks every en-dash again, and hides the ones that need to.
Click ->here<- to download the script; save into your own User Scripts folder, and double-click to run. If I'm correct, it does not need anything special from CS3, '4, or '5 — theoretically it ought to work with either.
.
