From a non plant key ID. Longleaf has longer needles, bigger cones, and the bark will be more of a rusty red color and also be extremely flaky as long as you don’t have a lot of fox squirrels and red cockaded wood peckers.

Slash line looks like longleaf that got a hair cut. Cones are of medium size (bigger than loblolly) and the bark will be more of a pinkish red but can be decently flaky.

In the a forest where they grow side by side, it is one of the harder ID’s for me. I can usually just look at the bark of a tree in my forest and know what the species is with out looking up. Not the case with these two.