Detection: Detecting whether a cell contains a particular sequence of DNA usually depends on use of a molecular probe. It can be an antibody to a protein coded by a particular DNA sequence. A cell containing that sequence of DNA may produce the protein and such a bacterial colony can be detected with an antibody against that protein prepared in rabbit or goat. Since DNA sequences are complementary, if you already have a sequence of DNA in which you are interested, you can use some of this DNA labeled with radioactivity as a probe to find bacteria with exact or similar complementary sequences to which the probe will bind.

Putting all the steps together:

Transform cells with the recombinant plasmids. Screen transformed cells with appropriate probes. Select colonies which have the DNA of interest.

General resource on cloning DNA