Unless you know something about comics and other similar type items you can generally look for the upc and give up on it of being extremely valuable unless it has some sort of uniqueness or rarity.
But in this day and age it seems every item at a garage sale has been checked on ebay before they ever put it out for sale
Still a cool find for 50 cents. Never really know, perhaps someday you might run into a hulk fan or someone in needs of that comic. For now atleast you can sit back by glow of the warm silver great finds you have had at garage sales and read a good comic