I'm puzzled by the term "offer" as well. From the comments below and no response back, it is likely that we're all simply 'fishing' with responses and assuming you're a FSBO. If so, an appraisal or fee valuation is one approach, but as a listing agent, I can tell you numerous stories of appraisals that had no basis in comparison to the price the home sold for. Just ask anyone in Austin who has had an issue getting a house to appraise when it goes with multiple offers and over list with limited comparable sales nearby or recent.
If your question was of the other variety, I concur with others that there is no 'validation' of offers by Trulia and listings are all (100%) listed in the MLS by licensed brokers.
Hope this helps.