So I flew for the PNW airline that pioneered RNP for a bit recently, and I was able to meet and befriend one of the gentlemen there who designed the RNP approach system back in the '90s. He and his colleague were responsible for all of the design and proving runs for RNP. He was teaching a class and this question came up. He said, and this is mostly a paraphrase: "there is no way for the pilot to determine when the fmc goes into approach logic (geometric vnav). It is buried in the coding, and for whatever reason, the pilots are not made aware when this change occurs, and it is different for different approaches." Take that for what it's worth, but from what I have learned from people who truly know this system, it isn't for the pilots to know.