How to display unroutable tracks early in eagle autorouting process
I don't think you can tell that there will be unrouteable tracks, or which tracks won't be routeable, until you've tried to route the whole board.

Which tracks are unrouteable, and how many are unrouteable, will depend on the order in which you (or the autorouter) routes things.

(edit)
If things appear unrouteable, moving components around may make the board routable.  With experience, you can see, while placing components, where you may have routing problems, so you can shift things to avoid those problems.