Your ability to determine the exact worth of an object depends entirely on how much information you have. It is never enough to just look at something and guess -- you must know what to look for regarding its condition and what similar objects are valued at. Sometimes this means taking older or seemingly rarer pieces to an appraiser, who can determine their authenticity and value. Other times, you can estimate yourself after carefully inspecting an item.
Carefully inspect your object for signs of wear and tear. This may include chips, flaws, evidence of touching up, stains, blemishes in the finish or tears.
Check for any indication that the object is a reproduction. This may include an imprint of the manufacturing date or a copyright.
Look up your object in a pricing book -- these are updated annually or every few years with new values. For example, you may find the value of an old action figure in a book that catalogs action figure prices.
Check websites like eBay for like items. For example, if you have a first edition of a classic book, check online auction sites or retail sites specializing in rare books to see what other people are charging.
Visit an appraiser with your object. Be sure that this appraiser is qualified to appraise your specific object -- those who specialize in furniture may not be qualified to appraise your antique silverware or baseball card collection, for example.
When searching for an appraiser, contact an organization such as the International Society of Appraisers. Groups like this require members to be formally trained and tested, ensuring that they are well-qualified to appraise your belongings.