Try using reflectance imaging to find the surface. You’ll need to set one of your detectors to detect laser light reflecting off the interface between the glass surface and the sample. Your confocal system may not have an option to set this up automatically, so you may need to do it manually. Just be very careful because the laser light is very intense and you don’t want to fry your detector (use lower laser power, low gain, low pixel dwell time, etc.). As you adjust your z, you should see an intensity maximum at the air-water interface. Note that you’ll also see an intensity maximum at other refractive index interfaces if you move too far out of focus (e.g. glass/oil, glass/air). Good luck!