Don’t worry, it’s not the “whiff” test.
“Sniff test” can actually refer to two different phenomena. It could be a test used to see in real-time if a patient has unilateral diaphragm paralysis–sniffing is essentially the opposite of a Valsalva maneuver and you would expect both halves of the diaphragm to flatten appropriately. If they don’t, that’s a problem.
It can also refer to duplex ultrasound, and trying to identify the subclavian vein. Eyeballing veins can be tricky. But if you ask the patient to do a “sniff test,” the subclavian should collapse, which can be a helpful way to identify it. Conversely, if you ask the patient to Valsalva, the subclavian should expand.