Check out this thread UC Davis has a test for splash publicly available
For some more info on Splash.
When breeding paints, and NdAppy and others have pointed out, there is always the chance of a minimally marked foal. It can happen with tobis, frames, splashes, etc. That is why I don't believe in "color guarantees" It would be better worded as a "copy of ___ gene guarantee"
The "expression of white" is something that science is yet to be able to fully predict, but hopefully someday!