The engineer's method: there's a way to wipe all markings and still figure it out. If the compressor is off and turns on when you turn the knob clockwise, then clockwise sets it to a colder temperature. Conversely, if the compressor is on and turns off when you turn it counterclockwise, then clockwise still sets it to a colder temperature.

Keep in mind that there should be a lockout period where the compressor does not come on within a few minutes of turning off, so you can't just move it back and forth and listen to the compressor start and stop.