I've been working to give each race 3 unique buildings to add to the strategy of the game.  So I've made two changes to the Human Buildings. 

First, the AA Defense now detects cloaked units.  The detector, with the same sprite, is now a shield generator.  Shots that hit the shield will weaken the shield, and any units that get inside the shield can fire at buildings without the shield getting in the way.

The Watchtower will still uncover a large line-of-sight. (So don't ignore them.  The fog of war uncovered really is worth the money.)  However, the Watchtower will also allow you to, for a price of crystals, pick a spot which will uncover a small amount of fog-of-war, as well as detect units.  (This is like the Terran Sensor Add-On in Starcraft) This spot is fixed until the player pays more crystals and picks another spot.

but if the detector units are too easy to build, the clocking units will be useless(You can't rush then.) making it not-so-easy will be better.

In SC it was very easy with zerg, but only for your base, not when attacking. Overlords could do it, but they moved slowly. For terran you had to build an enginerring bay if I remember and then turrets or science vessels. For protoss you needed cannons. In SC2 for zerg it's a nightmare because overlords needs to be morphed into overseers and overseers requires a lair. You can also build spine crawlers/spores, though.

Often we forget to build detection in those games so it should balance out how easy it is to get detection.