As stated there are multiple smoke machines on the market designed just for this most shops will do this check for 30 -70 dollars in Utah. Soapy solutions are no good as the moisture will frequently swell and seal small leaks, and the solution would also be drawn into the motor which may be undesirable. Also be advised that some components could be damaged by too much pressure. I'm sorry I do not know what they make the smoke out of(but it smells really good, kind of flowery) and some of them have UV dye in them and some don't.
I took apart the smoke machine at work today and the cannister that produces the smoke gives no indication as to what is inside. sorry
An after thought, the modern automobile is fairly tolerant. I would think that most of the ideas you have would leave particles that are smaller than what the air filter would allow so they probably wouldn't cause any harm. I am a master auto technician so I do know a little about this.