When cloning a foreign DNA fragment into a plasmid there are a number of methods

When cloning a foreign DNA fragment into a plasmid there are a number of methods that allow the selection of recombinant clones such as cloning into a selectable marker ( in the case of pBR322) or cloning into and interrupting the -galactosidase gene (in the case of pUC derived vectors). The loss of function of these genes can then be used to identify those clones that have a foreign DNA fragment insert. However with bacteriophage lambda (?) vectors it is not necessary to do this yet one can readily distinguish the vectors that have incorporated the large fragments of foreign DNA from those that have not. How are the recombinant vectors identified?