How to properly distinguish a transversal valley from a longitudinal valley?
I think that the simplest way is to just look at the map for these two valleys in the Switzerland:
Longitudinal valley:

You can see that this is perfect example on the map: we can easily see the direction of the ridges and the valley is pointed in the same direction. This type of valley was not made by river, but it was made with compressing of tectonic plates.
Transversal valley:

You can see here that some river got through the ridge (then they built the road because the river made all work). It doesn't go parallel ridges like the longitudinal valley. This type of valley requires a lot of work for the river so transversal valley are typically pretty short.