You typically take a sales assessment test after a first interview to see if you "have what it takes" to be a successful salesperson. These tests are often ordered through third-party services and measure your ability to think on your feet and effectively sell a product. While it is impossible to predict which sales assessment test your potential employers will use, you can prepare so that you won't be going in without knowing what to expect.
Study your past sales experience. Write down, or have a former co-worker or boss write down, your weaknesses in the sales arena that you will need to focus on. You can also look over old career performance reports to get an idea of where you stand.
Assess yourself and your skills using a marketing aptitude test, such as Central Test's Marketing and Sales Aptitude Test. This will allow you to pinpoint your weaknesses.
Practice your weaknesses by role-playing sales situations with friends or family members. Ask them to tell you after your role-play session how likely they would be to buy a product from you.
Browse sales assessment tests online. Many companies that offer sales assessment materials, like danjoy.com and salestestonline.com, also offer free trials or tests for companies. Take advantage of them and see where you stand.