I think it's interesting how everyone is so different in this area.
From every book I've read concerning this, if you're consistent with the way you check your CM, it will be accurate for you because you begin to see your own unique patterns after about the 3rd cycle of doing so.
Personally, as I don't typically produce copious amounts of EWCM, I check internally so I can catch it when I do have it. Otherwise (and I have tried this because I don't necessarily enjoy the science of sticking my fingers up there and inspecting things,) I don't notice very many accurate sensations throughout the day as DH has a way of making me feel particularly amorous without warning, and that's an awful time to note CM. LOL!