Finding air wires in Eagle
I can think of three options:


Zoom out as much as you can then use the route tool on the tiny board, this catches the air wire, then zoom in again and route it.
You can also disable the top and bottom layers so the air wire becomes more visible.
Yet another option is to run the provided "length.ulp" script (File->Run... or ULP button). This script shows a list of all the nets, on that list there is a column "Unrouted", some net is not completely routed a value should appear here instead of "--". You can then type on the command line "show net_name" to highlight it.