Spoiler alert: There is no easy answer to this question. However, lucky for you, you're on one of the best sites for giving you reference for what an action figure is worth. Just like any hobby or passion, to maximize your collection's worth, you'll need to put in the time through research and patience. Just be careful about thinking only in dollars and cents and don't forget that action figures have worth in the time you spend finding, understanding and collecting them as well. While it's nice to make some money on your hobby, you might find it more satisfying to help out fellow collectors. "Its the not the Destination, It's the journey."
Similar to the real estate market, a great way to gauge the worth of an item is knowing what similar items have sold for. ActionFigure411 does that for you automatically by collecting, cataloging and filtering eBay auctions to provide a very good reference of the going rate of an action figure (details). We also show the complete price history graph so you can see if the value is going up, down or just leveled off. Other ways you can find the value of an item is to search eBay's sold auctions (filter on Completed and Sold Auctions). Do not use what items are selling for as their worth because anyone can make up a price, but it doesn't mean someone will actually buy it. Mercari, similar to eBay, can also be used to find recently sold items.
Condition matters for just about anything you will purchase, especially action figures. This is especially relevant when buying vintage figures in their original packaging. The fading of the artwork, clearness of the plastic bubble, dents, dings or damage all weigh in greatly to the value of an action figure. Some figures are graded, which basically means a 3rd party independently provides a score to the figure. Higher graded items will obviously be worth more. Grading also helps ensure the item is authentic.
For loose items, the completeness of an item will hold the most value. A vintage Darth Vader that includes his original lightsaber and cape will be worth more.
Another simple question, but not easy to answer. Typically, anything that is rare coupled with a motivated buyer will provide value. You could own the only He-Man with two right legs, but if nobody wants it, then it won't be worth much. This might sound counterintuitive, but sometimes the things that nobody ever expected to collect, become the most valuable items. ActionFigure411 provides great stats showing the most valuable figures in a genre. These are updated daily and you can browse by: Star Wars, Marvel, Transformers, G.I. Joe, Masters of the Universe, Power Rangers, Teenage Mutant Ninja Turtles and DC.