How can I find the value of older US stamps which are not imprinted with a value?

HOPEFULLY THEY ARE SEALED! TAKE THEM TO ANY MAIN OFFICE OF THE US POSTAL SERVICE. THEY KEEP RECORDS AND COPIES OF EVERYTHING THAT THEY HAVE EVER SOLD. THEN TAKE IT TO SPORT MEMORABILIA, TRADING CARDS ESTABLISHMENTS. OR GO TO THE LOCAL LIBRARY AND LOOK IT UP IN ONE OF THEIR MANY COLLECTORS BOOKS AND OR TO A BOOK STORE. THEY CARRY EVERYTHING. BEST OF LUCK!