Methods for checking a connection between a compressed air outlet of a blood treatment apparatus and a pressure measuring line of an extracorporeal blood tubing set. The methods include providing a blood treatment apparatus having a compressed air line. A compressed air device is in fluid communication with the compressed air line for generating pressure within the compressed air line. A compressed air outlet is in fluid communication with both the compressed air line and an exterior of the blood treatment apparatus. The compressed air outlet is connectable with a pressure measuring line and a pressure sensor. The method also includes building up a pressure and an air flow in the compressed air line and/or at the compressed air outlet, measuring a pressure at the compressed air outlet or in the compressed air line, and evaluating an increase of the measured pressure.