Look at your windows at night (I'm serious.)

I ate dinner tonight at one of the restaurants that has me clean its windows once a week. This downtown Athens, GA restaurant has very large windows. Some of the largest windows in Athens.

I tried to hide my glances, but I peered up periodically to see if I could locate any areas that required a touch-up. I couldn't find any, but I digress. How can you test to:

A) see if your window is already really clean, or
B) determine if your windows need a good cleaning.

The test of a really clean window is to look through it at night. Look from both sides of the window (the inside and the outside . . . but you knew that). When it is dark outside, the inside lighting will often reveal any marks or dirt on the windows. Simply move window treatments out of the way and look at your windows. You may see plenty of things that don't belong there. Then walk outside when it is still dark and look into the lighted windows. Any smudges or dirty build-up on either side of the glass should be easily seen.