How to find wires of a given width in Eagle
1 - Type "DRC" in the command line.

2 - In the "Sizes" tab, change the "Minimum Width" size to larger than the width of the traces you want to change.



3 - Do a DRC check. On the "DRC Errors" window it will show you "width" errors and the layer they are in. If you check the "centered" box, it will center the trace on the editor window when you select a specific error instance. 




This does not answer your broader question of "how to highlight specific trace widths" but solves your problem of finding traces narrower than 'x' mil/mm.