Out of my current Facebook friends, how do I check which people added me as a friend and those who friend requested me first?
Is that possible to find out? And if yes, how?
For your first question: Who added you as friend, it's easy:
Now for Second Question:
There are two Scenarios:
You can't find out on Facebook for existing friends who sent the request. However, if you have e-mail notifications enabled for friend requests you could search for them among your e-mails and often find "NN confirmed you as friend" (or something like that, the wording has changed during the years) - then it was you who sent the request - or "NN added you as friend" (again, the wording has changed) - then it was NN who sent the request.
For exactly this purpose I have "friend list" called "Friend request" to which I add everyone that sends me a friend request that I accept.