Apparatus for causing a vacuum force to be applied across a width of a print medium as it moves through a printing system and for determining whether the edge seals are positioned correctly is disclosed. A vacuum chamber is connected to a vacuum source which provides vacuum to the vacuum chamber. Spaced sealing rollers and the edge seals define an opening in the vacuum chamber, wherein the print medium passes over the opening and is guided by the sealing rollers. A sensor measures a vacuum level in the vacuum chamber. A controller stores a plurality of response curves for different known print media, produces first and second signals indicative of an activation level of the vacuum source and the vacuum level in the vacuum chamber, and compares the first and second signal to appropriate response curves to produce an error signal indicating whether the edge seals are correctly positioned.