as far as im aware there is no way to stop a sniffer without manually removing it from one of his computers. however the fact he told you he could sniff you (that sounds so stupid

) probably means he isnt as he has now arroused your suspicions. to test i suppose you could tell someone in an IM chat something and then talk to him about it but dont mention what you told the IM person to him and see if he lets his guard slip and brings it up into the conversation.
regards
john