In today's market with the Home Valuation Code of Conduct in place, we are all stumped. Even appraisers are getting second-guessed by the lender's underwriters (all the time).
That said, it takes a good understanding about where appraisals are coming in around your property to know what to expect. As others point out, that is tricky. If you are selling a home, I would strongly suggest a response to any offer with an appraisal contingency include a provision that the buyer deliver a copy of the appraisal report to you before they can walk, etc. While you might not like what is in the report, as a seller you need to know what is going on.
We are seeing as much as 40% of transactions fail to obtain appraisals that reach the value that buyer and seller have agreed upon. The shortfall ranges from 3-35% in these transactions. You or your agent can figure out how much more cash a buyer would need to come up with to fill the gap. Then, be prepared.
If you are buying, you have the advantage of using the appraisal to get a fair deal. Fact is, sellers need to understand that is the reality today. Even cash buyers will be using the appraisal to ensure that sellers are realistic.