I knew a glass tube used to generate a sulfuric acid mist.  Also, people sometimes use cigarrete smoke as easy solution, but the smoke doesn't works very good, because the gas is less dense than air.

I know/ knew nothing about fume hood containment and your question prompted me to google for info. There are a number of informative hits. Did you consider the gases listed in the web link given, like ethylene or methyl acetylene? The attached web link (to a PDF) has some useful gas equivalencies and suggest N2O to "be an appropriate substitute for Sulfur Hexafluoride." I hope google and some top researcher here in RG will give you the required info.

http://ateam.lbl.gov/hightech/fumehood/students/su00/Fox/FHAlt_testing.htm

http://homer.ornl.gov/sesa/sustainability/fewg/bnl_chem_fume_hood.pdf

Dear Mr. Biffi, I think there is no perfect fit which will meet all aspects. I think Mr. Venkat Ramani's research in this field is very good. In case you want to have a substitute to sulfur hexafluoride, ethylene gas may also work. Gases used must be detectable even at low concentrations, non-toxic or have a very low toxicity and the density should be slightly higher than air. Then you can do a test according to the legal health and safety rgulations of your region or home country. Keep in mind that this is a test method which is correct only for the kind of chemical you have used within the linits of the test. It can but must not be correct for the chemicals you are working with. If you want to see how your vent system is working you may do it like this: turn off the ventilator, put in a flat vessel filled with a few ml of concentrated, aqueous ammonia solution or a paper towel wetted with it, close your hood and go for a coffee. Then you  quickly open the hood, put in a flat vessel filled with concentrated hydrochloric acid inside, close the shield immediately and wait a few minutes. All of the hood will be flooded with Ammoniumchloride dust. Then turn on the ventilator of the hood. You may directly see what happens. You can repeat this while the shield is opened. And what happens if a window or a door is opened or all other hoods connected to yours are working with opened shield or closed shields. If you feel this is interesting and want to learn more you may use a little (1 or 2 ml should do it!) liquid bromine at the beginning and then place a vessel with concentrated liquid ammonia inside instead of  hydrochloric acid. The dust will behave differently.  - If you want to learn more about the safety of the hood you are using and a total shift working nearby there is only the option of wearing an equipement that is also used near a production site, to find data of a local concentration of a dangerous chemical. Usually most of the dangerous chemicals will not kill you at concentrations of less than a few ppm (and in this case you should use a glove box!) This devices will collect the chemicals you may have contacted or breathed during your shift. Well, it will take some time to analyse the results and will cost some money. During 8 hours you will breathe app. 8 - 12 m³ of air. This means at an average concentration of 10 ppm you have inhaled 80 to 108 ml of a potentially dangerous chemical vapour. A very rough estimation will give 4.5 mmol, which you may multiply with the molar mass of this substance. Keep in mind that there is a difference in inhalative and oral toxicity. At a molar mass of 200 g/mol (well the sustance must be volatile at room-temperature) you have inhaled 900 mg. Toxcity is usually given in mg/kg of weight. Supposed you are weighing 70 kg - the toxicity of your substance must be app. 13 mg/kg . Well, usually you can see only LD 50 (which means 50 % of the animals are killed by the concentration given). As a very rough estimation you may double this number  26 mg/kg. Looking in the safety data sheets of the chemicals you are working with may be helpful.  In case of hydrogencyanide molar mass 27 g/mol, 10 ppm means 122 mg or 1.73 mg/kg - this means it will be really dangerous even fatal if you ignore all symptoms comming up within these 8 hours! Please keep in mind this is not a scientific "reporting" from my side. It is more or less a practical guide for daily and fast risk assesment that a plant manger or lab manger may need to do.

I hope it has been helpful and that our scientific community will not be too harsh on me concerning such a type of calculation.

Best regards, Dr. Wolfram Eichinger