To help you determine whether a package has dangerous goods, you should read the general descriptions on either the customs declarations or outer packaging of commodities being offered for mailing.

Non-exhaustive list of commodities that may have dangerous goods

Commodity Examples of dangerous goods which may be present
Agricultural supplies Fertilizers, pesticides, herbicides
Automobile parts Wet batteries, shocks or struts with nitrogen, air bags
Breathing apparatus Cylinders of compressed air or oxygen
Camping gear Flammable gas or liquid, matches, other dangerous goods
Chemicals Often found to be dangerous
Chemistry kits Dangerous chemicals
Cleaning fluids Dangerous chemicals
Cosmetics Hairsprays, flammable liquids, other dangerous goods
Cryogenics Very low temperature liquefied gases
Cylinders Compressed gases
Dental supplies Resins or solvents
Diagnostic specimens Infectious substances
Diving equipment Gas cylinders, high intensity diving lamps
Electrical equipment Mercury in switch and electron tubes
Electrically powered apparatuses (wheelchairs, lawnmowers, and so on) Wet batteries (corrosive)
Fireworks Explosive substances
Frozen fruit and vegetables Dry ice
Household goods Paint, aerosols, bleaching powder
Individual Meal Packs (IMPs) Flameless Ration Heaters (FRHs) and Self Heating Beverages (SHBs)
Instruments Barometers, manometers and sphygmomanometers containing mercury, mercury switches, and thermometers
Laboratory equipment Dangerous chemicals
Machinery parts May include adhesives, paints, sealants, solvents
Medical supplies Dangerous chemicals, thermometers, blood pressure instruments
Paint Flammable solvents
Party supplies Compressed gases, explosive articles
Perfume Flammable liquids
Pharmaceuticals Dangerous chemicals
Photographic supplies Dangerous chemicals
Repair kits Adhesives, paints, organic peroxides, solvents
Samples for testing May have dangerous goods
Swimming pool chemicals Dangerous goods
Switches (electrical) Mercury switches
Tool boxes May have explosives (power rivets), aerosols, adhesives
Toys May be composed of flammable material
Vaccines May be packed in dry ice