How to determine whether a large container is air-tight? In constructing a kitchen-waste digester at home, I use a 50 Litre HDPE drum. The base of the drum is holed with a plug fitted to allow drainage when necessary. 
The top has two openings - one for inlet, the other to act as outlet for the generated fluid CO2/CH4.
The drum is to be used filled to about 2/3rds lying on it's side. Therefore any leakage at the drain will be immediately visible. The same applies to the inlet pipe - it too shall be filled with water. 
A heated iron nail was used to make the outlet hole. Then a ball-point pen was inserted into it; a very snug fit. I then melted a silicon glue-stick to attempt to seal any leaks at this point.
The problem at hand is to determine whether the outlet is air-tight. It is possible to dunk this container in water and test for bubbles; but the same test will not be possible with a larger container (300 litres) to be used if this experiment is successful.
Suggestions, anybody?
 A: Similar situations can arrise in experimental work on large scale machines, and there is a body of knowledge that gets passed around.


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*The "can it hold pressure" test suggested by other answer works best if you can apply a pretty good over-pressure and have either a sensitive pressure gauge in the system or can afford to wait over night to see how tight your system is.

*You can buy comercial "bubble leak test solution", or bodge one together with clean water a little dish soap and glycerin. The commercial ones will be more sensitive. In either case, you generate a slight over-pressure and then apply a little solution along seams and joints. If bubbles form you have a leak.

*A very well equipped lab may have a "sniffer" a small device for detecting trace amounts of some particular gas in the air. Fill the vessel with a working pressure of the gas the sniffer detects and run the sniffer slowly along seams and joints.
A ordinary cigarette lighter will function as a low sensitivity sniffer for helium as the flame will turn blue in the pressence of the gas. This trick is known to old particle physicists who build wire and proportional chambers (helium is safer to test with than the often flammable working gasses and if it holds helium it will hold anything). However, it is essential to know that the chamber is filled with helium before doing this because many of the working gases (such as ethane-argon) are explosivly flamable, and finding a leak with a lighter can destroy the windows of the device and put you and your colleagues at risk (I heard this warning from a guy who burned the windows off a 6 cubic meter MPWC this way ...).
A: If the container is airtight, it should get more and more difficult to pump air into it. If this difficulty (i.e. the pressure in the tank) does not rise at all or flats out after a short time, you can be sure that there must be a leak somewhere.
However, take care not to increase the pressure too much, as that might break the sealing.
A: A 300 liter container might not fit completely in a bathtub, but you might submerge different parts of it a different time and then check for bubbles, depending on its overall shape.
Or you can just fill it with water and see if it drenches.
