I've used smoke pellets for testing air flow in cases a few years ago.
You can get pellets for flue testing which last from 30 seconds up to 8 minutes, & in various colours.
I used a piece of perspex temporary fixed & sealed in place of side panel on a case.
My setup was outside,case on a table with the pellet in front, which I light, then placed a 4" 90 degree plastic soil pipe bend over the pellet to direct smoke through front of case.
A lot of smoke is produce, so stand well back or you cough your guts up with some of the pellets.
I found pellets easier to use compared to matches, as I used to stick matches into some blue tack, & as they burned, they fall a part.
Never got around to posting videos on utube though.
Don't use smoke bombs, THEY will stain your case.