$8B holds currently-pressed keys. AND with the below to get the player's customizable keys.

7E:09AA Up
7E:09AC Down
7E:09AE Left
7E:09B0 Right
7E:09B2 Shoot
7E:09B4 Jump
7E:09B6 Run
7E:09B8 Item Cancel
7E:09BA Switch Item
7E:09BC Aim Up
7E:09BE Aim Down

$4218 (Saved to $8B also): Joypad #1 status register (1=pressed)

8000: B button
4000: Y button
2000: Select
1000: Start
0800: Up
0400: Down
0200: Left
0100: Right
0080: A button
0040: X button
0020: L
0010: R