The U.S. Treasury first issued silver certificates in 1878. These first certificates are rarer. Other rare years are 1880 and 1886. Look for the date on the front of the bill, usually accompanied by the word "series" as in "series of 1880."
Silver certificates with serial numbers below 100 are rarer and more valuable than those with higher serial numbers. If the serial number begins with a star instead of a letter of numeral, that is also rarer and considered more valuable.