Easiest way is to post a picture of it here and someone will tell you. Other than that, the main things you want to look at are: 1) Condition. This is probably most important. Does the bill look uncirculated? Does it have a few folds, but look otherwise like new? Is it well worn? 2) What is the series? 1950 E notes have the biggest premium, followed by 1950 plain at a distant second. 3) Serial number. Is it particularly low? Is there a 'star' following the serial number instead of a letter? 4) Fed district. Which of the 12 fed districts issued the note? This means very little in determining the value of 1950 series $20's, but can make a BIG difference in some instance.
