Usually in determining gene KO, you need to identify a section of your gene that will be present in the wild type but absent in the KO in the PCR product. This PCR product should be generated by the same pair of primers for both conditions.

You may identify this absence either by identifying a shorter PCR product in the KO compared to a known wildtype or by the elimination of a restriction enzyme site in the sequence to be deleted. In this latter case, the RE should cut the sequence in the wildtype but not the KO.

If your mouse is wildtype homozygous and you have generated a single PCR product to genotype it, you will see one strong band of the size you expect your wild type PCR product to be. You can determine this size by looking at the distance between your primers on the gene itself (Genome Browser or NCBI pick primers tool).

If your mouse is KO homozygous, you should see one strong band of the size (smaller) you expect your KO to be.

If your mouse is heterozygous, you should see two bands: one at the wild type size and one at the KO size.

Similar principles apply for the RE test, but consider the sizes you expect to get out: if you are looking for the KO to eliminate the RE site, you will be looking for one band in the KO, two bands in the wild type, and sometimes three bands in the heterozygote. 

Hi Rienecker , thanks for your answer. It really helped.