A method of detecting transfer leaks within a solid polymer electrolyte fuel cell stack comprises: supplying fuel or inert gas to the oxidant manifold(s) of the stack at a first pressure; supplying the other of fuel or inert gas to the fuel manifold(s) at a second pressure greater than or equal to the first pressure; optionally supplying fuel to the coolant manifold(s) at a third pressure; applying a potential difference across at least one of the fuel cell assemblies; and measuring the transfer current across the fuel cell assembly or assemblies. A corresponding apparatus detects transfer leaks within solid polymer electrolyte fuel cells stacks.