The Ghost Sniffer was said by Peter Venkman to be one of the they little toys. However, this device samples the air for ghost activity as it is a form of detection equipment. In reality, it is a Bacharach model 303 air testing device, it analyzes the particles in the air.
This device appears twice in the first film, once with Ray in the ad "We're Ready to Believe You" and with Peter while checking out Dana's Apartment.