Recent experiments to test Bell's inequality using entangled photons and io
ns aimed at tests of basic quantum mechanical principles. Interesting resul
ts 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 ro
le in verifying atom entanglement schemes. We describe as an example a sche
me 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. D
uring the state preparation no photons are emitted, and observing a violati
on of Bell's inequality is the only way to test whether a scheme works with
a high precision or not.