How does one know they are having a decrease in milk supply?

My little guy is sleeping through the night from 11 PM until about 5 AM or 6 AM, and has been having about 1 - 1.5 ounces of formula through the supplemental nursing unit from medela with his last feeding @ 10:30 PM. My breasts aren't engorged, and rarely feel engorged (anymore) in the AM. Lately he's been playing at the breast, but he's been nursing every 2 hours the last few days (he was nursing every 3 hours prior to this week). He's also been spitting up a lot, but it's very watery. We've also been treating a case of thrush, I'm taking Diflucan. His diapers are wet/soiled as they have been, but I'm concerned that my milk supply is decreasing.

Any thoughts on this?