As a former patrol division deputy sheriff in Santa Fe, New Mexico, I, as well as any other officer who has conducted countless DUI arrests, can fairly state that drunks often do not cooperate with our examination procedures. One problem that comes up is determining whether subjects are really blowing hard enough to submit a sample into the Intoxilyzer Inc. AlcoSensor handheld device.
A useful and very visual way (especially nice when used with an in-car video camera to record the process) to determine if they are blowing hard enough is to remove the clear plastic wrapping used to protect the mouthpiece prior to use. Hold the normally discarded wrapping vertically in front of the subject as they blow into the mouthpiece. Assuming that it is an AlcoSensor mouthpiece that does not have a restriction at the far side from the subject and gives minimal back pressure, there should be a free flow of air through the mouthpiece causing the plastic wrapping to flutter.
If the wrapping material doesn't flutter, you can question the subject's cooperation in submitting a breath sample.