The only way I know of to test the T-stat is to remove it and hang it into a pot of water. As the water begins to heat up and boil, it will open up. Then you watch to see if it closes when heat is removed.

It sounds like someone has messed with the idle before as turning it counter clockwise (out) is suppose to increase overall idle speed. It could be that there is something wrong with the thermo valve in the throttle body.