How do you really know that that great item you just found at the neighborhood garage sale is the real deal? In other words, is it really a collectible item or someone else's discarded piece of junk?
What's its condition? Does it look new or used? Clean or dirty? Scratched, torn soiled, or nice, new and pristine? Did your new doll, train or other unique item come with the box or without?
Here's an idea for establishing its heritage: look carefully for any brand names, place of manufacture, trade or any other type of mark. How about when it was made. Some treasures will have a date or at least a year stamped on them. This can help you verify it's true background.
If the item is a doll, is it still in its original box, or does it even have the box at all? When inspecting your doll, look for the above mentioned identifying marks.
Use catalogues, hobby magazines and online groups to help you know the value of your collectible. Online auctions can be a great source to see what similar or the same items are selling for. This is a great indicator of the approximate value of your item. Just remember, you can probably get more for your item if you sell it locally where the prospective buyer can see, feel and touch the collectible.
Finally, be sure to visit the local antique shops and see what they will give you for it. Some areas even have hobby clubs and organizations where you are sure to find help with your item's value.