As a general rule in California, someone who has stipulated to paternity lacks standing to bring the nonpaternity action. Once the stipulation is reduced to final judgment, the paternity issue is not subject to relitigation. DNA evidence is immaterial. The court would be uninterested in the surreptitious test, just as it would be disinterested in ordering tests.

There are some very limited exceptions. For example, if executed by a minor, the voluntary declaration creates only a rebuttable presumption of paternity until 60 days after both parents reach age 18. It may otherwise be nullified or set aside in a few circumstances, such as a court-ordered DNA test requested before the child's second birthday. However, the judge still can deny the set-aside by finding that it would not be in the child's best interest. This can be based on anything the judge desires, as long as it is explained. Factors include whether the nonfather has made it difficult to establish the biological father, or to get support from him, or whether it would just be a bummer for the kid. If the nonfather has no real contact with the kid, and a good substitute is available, it may be do-able.

I'm not sure about the surreptitious testing. I suspect that everything above matters much more. I suspect that, absent some testing law I don't know about, a "chewing gum test" could be used as a starting point to get the ball rolling.