A B737-700 PASSENGER DEPLANING (PILOT FROM ANOTHER AIRLINE) INFORMS FLIGHT CREW HE SAW DENTS ON THE SPOILERS. CAPTAIN MENTIONS THIS TYPE OF DAMAGE CANNOT BE SEEN DURING NORMAL WALKAROUND.

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PICKED UP AIRPLANE AND FLEW TO ZZZ. BLUE SKIES THE WHOLE WAY. PAX DEPLANING IN ZZZ (A PLT FROM ANOTHER AIRLINE) MENTIONED HE SAW SOME DENTS IN THE SPOILERS SO HE AND I WENT BACK TO LOOK. CALLED MAINT WHO SAID THEY HAD NO RECORD OF IT. BY THAT TIME LINE MAINT WENT OUT ON THE WING AND SAW A LOT OF DENTS. ACFT WAS TAKEN OTS. MY QUESTION IS: THESE COULDN'T HAVE BEEN SEEN ON A WALKAROUND; SO HOW WILL WE KNOW IN THE FUTURE? IS THE MATERIAL THE SPOILERS ARE MADE OF SO MUCH SOFTER THAN THE REST OF THE WING THAT THEY SHOW DAMAGE WHEN NO OTHER IS VISIBLE? SUPPLEMENTAL INFO FROM ACN 770694: WE DID NOT ENCOUNTER ANY ADVERSE WX CONDITIONS THROUGHOUT THE ENTIRE FLT FROM OUR DEPARTING CITY. TO THE BEST OF MY KNOWLEDGE THE FLT SPOILERS WEREN'T USED EITHER. IN OTHER WORDS; THIS DAMAGE MOST LIKELY OCCURRED SOMETIME BEFORE WE TOOK THE ACFT. SINCE THIS WAS NOT PREVIOUSLY RPTED BY A FLT CREW; THE DAMAGE PROBABLY OCCURRED WITHOUT THEM NOTICING IT. HOW OFTEN DO MAINT PERSONNEL CHK THE TOPSIDE OF THE WINGS FOR DAMAGE?