If the property is in New York State, you can find out how much comparable houses in your area sold for recently from an official records search of the County Clerk's Office and the County Real Property Tax Services. Sometimes this is on line, depending on the County, sometimes you have to go to the County offices to find that information.
Get the tax map parcel ID for the comparable property and then locate the deed to the current owner. The deed document will have a "tax stamp" on it with the amount paid for transfer tax, which is generally $4.00 per $1,000 sales price (ask at the Clerk's counter what the rate for that county is). Do the (simple) math. Certain intrafamily transfers might not have a tax, so you may have to go back to a previous arms-length sale deed which might not have a comparable price, depending on how long ago that sale was.
I've found the online services to be OK in terms of price data, but they seem to be based on assessed valuation for tax purposes which may be "off" because of people protesting assessments, age of assessments, etc. Assessments usually track the most recent sales prices though, so sometimes the online realty services are in the ballpark.
The trick is choosing the comparables. I'm not recommending realtors necessarily, but they do have insights when making listings as to what the comparables are (bedrooms, baths, square footage, location, amenities, etc.) so you have to be confident in your ability to identify recent sales in your neighborhood which are similar to your house.
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The County Clerk's Office.
I am a former federal and State prosecutor and now handle criminal defense and personal injury/civil rights cases. Feel free to check out my web site and contact me at (212) 577-9797 or via email at Eric@RothsteinLawNY.com. I was named to the Super Lawyers list as one of the top attorneys in New York for 2012. No more than 5 percent of the lawyers in the state are selected by Super Lawyers. The above answer is for informational purposes only and not meant as legal advice.
The County Clerk's office has reliable information so long as you know the property address that you are researching. There is a periodical called "Comps" that your public library may have. Some counties like Nassau, have all sales on line. I don't know about Westchester
If you'd like to discuss, please feel free to call. Jeff Gold Gold, Benes, LLP 1854 Bellmore Ave Bellmore, NY 11710 Telephone -516.512.6333 Email - Jgold@goldbenes.com