Suppose we have an antibiotic ampicillin. Now the DNA fragment is encoded with ampicillin resistant gene which we have as a recombinant gene. This recombinant gene is inserted into the individual. Now we have two cells, one which has ampicillin resistant gene and the other which does not have it. We will keep both the cells in ampicillin containing medium. Now the cell with ampicillin resistant gene would survive while the other cell which does not have it, would die. Proving that the one which survives is a recombinant one.
