Gather the materials. You will need a pencil, a pad and most importantly, what is known as a swami gimmick. To do a poor man's swami, which I suggest, break off a piece of led from a pencil and wedge it lightly under your fingernail.
Find a willing spectator and first ask if he has a pencil you can borrow. If not, use yours, but using borrowed items is always better in magic for believability sake.
When he hands it over, ask if he in fact, has a banking PIN number. You can even throw in some small talk about how he wouldn't want anybody knowing it.
Tell him to visualize the first digit of his PIN. Then the second. Then the third. Finally, the fourth.
All the while, throw him mumbo jumbo about eye-accessing clues and psychic theories. As he visualizes the numbers, you are going to PRETEND to write down something on your pad. Make it look believable, though.
When finished, confidently explain you have made your prediction. Have him say his PIN.
After he says it, with the pad and your led-finger lowered, misdirect with questions about how remarkable it would be if you got it right, then actually use the led to write the number he just said.
Reveal your pad to show you got it right!