The odour in jet propulsion fuel comes from sulphur compounds (mercaptans) which in very small concentrations have a penetrating, intense and unpleasant odour. Odours may arise in the handling of jet propulsion fuel and in exhaust through incomplete combustion. Mercaptans belong to the same group of substances that create odour problems in connection with paper pulp production (the sulphate process). Scientific studies show that mercaptans are not toxic at these low, odour-producing concentrations.

If you know what you're looking (sniffing) for, yes.

Avgas 100LL smells like gasoline (a little different than what you pump into your car, but similar).

Jet-A smells like kerosene (not quite 'like diesel', but definitely not like gasoline).

If you are familiar with how both smell you can tell them apart by odor, and a significant contamination of Avgas with Jet-A could be detected by smell. (Other tests, like an evaporation test, are more reliable for lower levels of contamination however).

The main reason for smell in airplane fuel is tanks filled with the wrong fuel

Recently, a small plane operating from a nearby airport suffered an engine failure on takeoff and crashed. One of the hazmat responders dealing with the resulting fuel spill mentioned 'a strong smell of Jet A', leading to speculation that the airplane had its tanks filled with the wrong fuel. Would he have been able to distinguish Jet A from 100LL by smell?

Natural gas is a fossil fuel formed when layers of buried plants, gases, and animals are exposed to intense heat and pressure over thousands of years. The energy that the plants originally obtained from the sun is stored in the form of chemical bonds in natural gas. Natural gas is a nonrenewable resource because it cannot be replenished on a human time frame.[2] Natural gas is a hydrocarbon gas mixture consisting primarily of methane, but commonly includes varying amounts of other higher alkanes and sometimes a usually lesser percentage of carbon dioxide, nitrogen, and/or hydrogen sulfide.[3] Natural gas is an energy source often used for heating, cooking, and electricity generation. It is also used as fuel for vehicles and as a chemical feedstock in the manufacture of plastics and other commercially important organic chemicals