Spraying a light oil into the intake (like WD40) will burn and create smoke in the exhaust. Its best to do it past the MAF sensor. If you have a leak or a crack in the manifold, it will show up easily. After you do the test you'll want to run the motor for a few minutes to finish burning off any excess oil so the plugs don't foul.
2000 F-150 XLT Ext Cab. 4.2L V-6 153K miles
1992 Explorer XLT 4x4 5 speed stick 229K miles
1970 F-250 Camper Special 390 Auto 293K miles