As Raiyn says. Use a spirit level. Make sure the floor the bike is on is level first. Then test the level of the seat. The likelihood is that the toothed adjustment won't give you a perfectly level seat. You also may need to make millimetre-by-millimetre adjustments to fine-tune the most comfortable position.

The problem may be even more fundamental, however. Is the seat height right? What about fore-aft adjustment of the seat on the rails? There are three things we deal with in our training courses when fitting bikes to participants -- seat height, seat fore-aft position, and seat level. Works for our participants.