Papers, Please

Papers, Please is an unmarked quest

Detailed walkthrough
Talk to Ralph at Mick & Ralph's at Freeside. After speaking to him about his special services, he can offer you something, but only after you have impressed The King.

Find a way to do that or continue with some further digging. This reveals Ralph is crafting counterfeit passports. Obviously he wants to keep this low key, so he offers you a passport for 500 caps, or with a Barter skill of 50, 375 caps.

Once obtained, use it to pass the credit check at The Strip North Gate.

Alternatively, complete the quest G.I. Blues as The King will then owe the player a favor.