Over the years I’ve got that question from a lot of people. I remember many years ago as a novice Sarracenia grower, mixing those two species up was VERY easy.
I have to admit, in full growth, they do look rather similar, but there a couple of fool proof ways to tell. One of them is now… at flower time.
Take a look at the two picture below. The one on the left is S. oreophila and the one on the right is S. flava. S. oreophila sends up its pitchers FIRST, then it flowers, S. flava on the other hand, sends up its flowers first, and then its pitchers.
As I said, there are a number of other ways… but this one seemed appropriate to mention as it is, at least here, flowering time!