How is it possible to tell if dangerous radiation is leaking into the environment from a radioactive source?

The radiation from a radioactive sources is usually in the
form of alpha particles, beta particles and gamma
particles.

There are several methods of detecting these
forms of radiation which differ based on the effects that the radiation have on
different substances.

The most widely used apparatus is
called a Geiger counter. This works on the principle that when a radioactive particle or
a photon passes through an inert gas, the gas becomes conductive. Electronic detectors
are able to measure this change in conductivity and detect the level of radiation. The
apparatus can be made more sensitive to a particular form of radiation by choosing an
appropriate gas or similar substance that changes conductivity when struck by
radiation.

Another apparatus is a scintillation counter.
Here, the detector is a substance called a phosphor that gives off light when struck by
particles of radiation. The light is detected using an electronic circuit and based on
the number of particles that strike the phosphor and their energy, is able to give an
estimate of the level of radiation.