A splinter is a ring R for which all finite ring extensions R -> S split as a map of R-modules. While this is a characteristic free property, it exhibits wildly different behavior in various characteristics. In this talk, we prove that under mild hypotheses, the locus of primes p in R with the local ring R_p a splinter is Zariski-open. In particular, we define a splinter test ideal, and show that this construction behaves well under localization to conclude that the vanishing set of this ideal is exactly the non-splinter locus.