How? In case of he has taken your pen, and telling that he has not taken and so on.

First ask him, have you taken my fountainpen? Easy way to identify, no tests, no questions, no observations, no reminders for getting the truth, just "count his pulse rate immediately as soon as he speaks with you", if some one is speaking or giving wrong information can caught by this simple process. Keep your 2 fingers on the wrist of your friend, count the "irregular pulse", when he is speaking lie then the pulse will run faster. Secondly if someone speaking false then his eyes will start flirting, and his eyes will not rest on any thing, it means he is speaking lie.

When he will caught he will laugh.

Note: our eyes never speak false.