A mailing system includes a mailing machine having an envelope feed path, a sealing system configured to apply a liquid to an envelope in the envelope feed path, a capacitive sensor located in the envelope feed path downstream from the sealing system, and a controller connected to the capacitive sensor. The sealing system may include a liquid reservoir and a liquid applicator. The capacitive sensor is configured to measure a quantity of liquid on a portion of the envelope applied by the sealing system and to generate a signal based on the measured quantity. The controller is configured to perform an operation based on the measured quantity signal from the capacitive sensor.