Well let's see, there are four types of proximity sensors; infrared, acoustic, capacitive, inductive. Inductive is out since nothing you are detecting is metal. Capacitive is also probably out. That leaves infrared and acoustic. The questions becomes if the substances you are trying to detect can reflect the infrared or acoustic light/sound waves?
Not having done any work with either sensor, my first gut reaction is think acoustically. As any substance on the glass would change how it would reflect the sound waves back to the sensor. How sensitive the sensor is though would effect how well it would work, as well as how noisy the surrounding area is. What exactly is the point of your project if I may ask?