  We propose a renormalization scheme that can be used as a reliable method to
identify universality classes of absorbing phase transitions. Following the
spirit of Wilson's block-spin renormalization group, the lattice is divided
into blocks, assigning to them an effective state by a suitable Boolean
function of the interior degrees of freedom. The effective states of adjacent
blocks form certain patterns which are shown to occur with universal
probability ratios if the underlying process is critical. Measuring these
probability ratios in the limit of large block sizes one obtains a set of
universal numbers as an individual fingerprint for each universality class.
