My understanding is that you can tell mountain laurel from what we call rhododendron based on the laurel's squared off or polygonal blossoms.
Last year the mountain laurel was in full bloom right at the trailhead to Lynn Camp Prong Trail (all the way up the dirt road past Tremont) during Troutfest! So that would be a great time to plan a trip.
Interestingly, 'rhododendron' is actually a genus (class of plants) that includes a lot of the azalea plants we all have in our lawns (me included).
There are actually many types of rhododendron and I suspect that includes the various kinds and colors we see in the Park.
Here's some mountain laurel (Kalmia latifolia) in bloom during Troutfest last year:
Now here's some rhododendron (I believe) getting ready to bloom out:
I have some blooming shots but none of them are on the net, so I'll borrow this one:
See the wrinkled, round edges on the flowers? The foliage is a little bigger and a little more rubbery, too, but not enough for me to really notice except when it's in bloom. Both are beautiful and abundant in the Park.