If you want to check air flow you need to use something like this:
http://testproducts.com/second-smoke...ack-p-242.html
Incense is not a good method, though it's cheap and seen around quite a bit. Your nose attenuates to the smell very quickly and you don't notice it. The smoke isn't very concentrated and dissipates quickly. Do the job right and use a professional product that is a standard in the HVAC industry as well as with Safety and Risk Management teams who evaluate chemical and biological safety hoods.
Robert