A stack (10) of fuel cells (11) is to be tested for tightness of the fuel cell membranes. For this purpose, a tracer gas is introduced into the fuel feed channel (15) of the stack (10). The fuel discharge channel (16) is either open or it is closed. A carrier gas is fed to the feed air channel. (17) and led through the exhaust air channel (18) to a gas sensor (28) where a determination is made whether the carrier gas contains amounts of tracer gas. A defective fuel cell (11) can be located by introducing a lance comprising a sniffing probe into the corresponding channel (18) and determining the position of the probe.