  Recent experiments to test Bell's inequality using entangled photons and ions
aimed at tests of basic quantum mechanical principles. Interesting results have
been obtained and many loopholes could be closed. In this paper we want to
point out that tests of Bell's inequality also play an important role in
verifying atom entanglement schemes. We describe as an example a scheme to
prepare arbitrary entangled states of N two-level atoms using a leaky optical
cavity and a scheme to entangle atoms inside a photonic crystal. During the
state preparation no photons are emitted and observing a violation of Bell's
inequality is the only way to test whether a scheme works with a high precision
or not.
