There are several reputable trading card appraisal companies, including: PSA/DNA, Beckett, GAI, among others. Some have more clout than others, depending on area of expertise (i.e., James Spence, autographed items).
Well you can go to a collector to see what your cards are worth. Another thing you can do is just list them on ebay or maybe check what the rates are for your baseball cards. Some people are willing to pay 100's for a certain baseball card over others.