In Papyrus, you want to return whether the player is in the first person camera. You'll probably find "GetAnimationVariableBool("IsFirstPerson") == 1", which at first glance looks okay to use. However, at some point you will receive reports from a few users that the mod has stopped working. I will explain why this issue occurs, why it doesn't happen for everyone, and the solution.

"GetAnimationVariableBool("IsFirstPerson")", this works, until the player enters and exits Vertibird. After that, "IsFirstPerson" gets stuck at false (i.e. third person) and, as far as I know, is only fixed by the player entering power armor. You can see it with the console command "Player.GetGraphVariableInt IsFirstPerson".

There are two solutions here, the latter is better but requires F4SE and Garden of Eden Papyrus Script Extender by LarannKiar.

1: "GetAnimationVariableBool("bGraphDriven"), this function returns 1 (i.e. true) in third person and always false in first person. However, in third person it returns false only while jumping. I've tested every different situation I can think of and this is the only issue I've found.

2: "GardenOfEden.Is3rdPersonVisible()", so false is first person. It works as intended. (Lighthouse Papyrus Extender by GELUXRUM has "Lighthouse.IsThirdPersonModelShown()". I've never used this, but I think it works the same way.)

I hope this article can help you.