Yes, you can test them at any time. However, we recommend doing cheek swabs around 4 weeks of age or when the pups can safely be isolated from mom and littermates for at least an hour.
When you do the cheek swab, please have someone help you hold them and give them plenty of water during the one hour wait. Insert the swab between the cheek and gum and press gently towards the outside, against the cheek. Rub and roll the swab against the cheek for a slow count of 30-45 seconds. This should collect enough cells for DNA extraction. The primary mistake that many people make is not leaving the swab in the mouth long enough to collect sufficient cells.
Note that while genetics will not change as the puppy ages, it can be very difficult to observe physical breed traits in young dogs because they are growing and developing rapidly. Most dogs will not achieve their final mature physical traits until they are at least one to two years of age.