Gel electrophoresis is used to visualize DNA following a PCR reaction. 

In the gel electrophoresis technique, an electric current pulls the DNA fragments in a sample through a gel matrix. A DNA-binding dye and a DNA ladder are also added to the sample. 

The dye allows researchers to visualize the stained DNA fragments as they migrate through the gel and the DNA ladder acts as a reference point, allowing researchers to determine the approximate size of the fragments in the sample.