The proper way is a pressure test. You could actually make one. All you have to do is pull the Air diaphragm and put a plug in the hole, then pull exhaust at furnace and plug it. The only thing you need is the gas flapper because you have to attach a 12" long nipple to it and put a gauge and shredder valve on. Pretty simple plugs range from 3/4" to 1 1/8"