How does one beat stealth units

I find it really hard to defeat players who use stealth units because of their invisibility. Any advice on how to deal with these? It usually seems like its a city for a city until either my or my opponet's capital is captured...

Build walls, use sentry planes to detect them.

Walls, sentry and stealth units placed just outside own cities... try to constantly move own units for possible stealth units detection.

Read this thread:

Proper use of Sentry planes: http://atwar-game.com/forum/topic.php?topic_id=3416