Normally the person who has knocked out the mouse, he has used some strategy to knock the gene out. Unless you do not know that, you can not know the primers and you can not design primers. One gene can be knocked out by many people and strategy could differ between different labs. Also if mouse is sold by Jax lab, each mouse has strain number:
For example: strain number of Dbx1tm1(cre/ERT2)Jcor/J mouse is
Stock No: 028131.
1. Now go to a site: https://www.jax.org/protocol/search#
2. Enter strain number 028131
You will see a page, scroll down the page,
You will see PCR protocol
Click that PCR protocol
You will see primers sequence and also how to conduct PCR,
All given on Jax site,
Besides you can design your own primers once you will get detail of mouse from Jax for this strain number
And similar for other transgenic and knockout mice