An attribute of growth behavior of eight bacteriophage mutants (1 to 8) is investigated in experiments that establish coinfection by pairs of mutants. The experiments determine whether the mutants complement one another  or fail to complement  These eight mutants are known to result from point mutation. The results of the complementation tests are shown below.

In each coinfection identified as a failure to complement (−) in the table, researchers see evidence of recombination producing wild-type growth. How do the researchers distinguish between wild-type growth resulting from complementation and wild-type growth that is due to recombination?