I wear a Rudraksha with 5 faces / Panchamukhi.
I read somewhere that there are certain mantras to be chanted while wearing it.
can anyone tell how can I know which one is associated with the Rudraksha I wear.
Shiva Purana Vidyesvara Samhita Chapter 25 describes about the greatness of Rudraksha.
For the five-faced Rudraksha (Pancha Mukhi Rudraksha) Shiva Purana states:
पञ्चवक्त्र स्वयं रुद्र कालाग्निर्नामत प्रभुः ।
सर्वमुक्तिप्रदश्चैव सर्वकामफलप्रद ।।
Rudraksha having five faces represents Rudra himself and is known by the name Kãlagni. It fulfills all the desires, bestows salvation to the devotees.
Regarding the mantra to chant while wearing five faced Rudraksha it states:
ॐ ह्रिं नमः
Om Hrim Namaha
As a sidenote Mantras are not supposed to be chanted and applied just because you see it here or in other places. One should have authority to chant mantra. So, I suggest you at least you read that chapter from Vidyesvara Samhita (Chapter 25), so that you can have authority from Shiva Purana itself to chant it while wearing.
There are also seperate mantras which are to be chanted while wearing Mãlã of Rudraksha or while wearing it in specific places. Like the use of PanchaBrahman Mantras. That chapter of Shiva Purana (Vidyesvara Samhita Chapter 25) makes it clear regarding recitation of other mantras.