Reading the real estate classifieds and checking on-line databases, such as www.nytimes.com, the NYTimes' classified site, will give you a broad idea.  But remember, this is all advertising.  They only tell you the good things about an apartment.  You’ll never see in an ad that the bar around the corner disposes of its bottles underneath the bedroom window at 4:30 a.m.  or that, although the bedroom has a partial river view, the livingroom is 10’ from a brick wall. 

Ask someone who has seen what’s out there and/or has a database containing that information.  While condo sales are available as a matter of law, co-ops are not recorded in any location, and real estate brokers do not automatically exchange that information.  But a good sales agent has the contacts to find out what sales prices have been and what’s on the market currently and the price it’s asking.

Any real estate agency will send you someone to do an appraisal.  Ask several, and ask them for 

• a list of comparable, closed sales
• a list of comparable apartments for sale (with square footage, maintenance, views, condition, other amenities).  A two bedroom, 4-room apartment can be anywhere from 900 to 1500 square feet or larger. 
• what they have personally seen that compares with your apartment

Don’t be swayed by an agent who sets a price that’s wildly higher than what other agents have suggested.  Ask for specifics on how they arrived at the figure. 

Go to some open houses to see for yourself. The NYTimes' classified site at www.nytoday.com has a list of open houses, as does the Sunday New York Times.

Talking to several real estate agents about the value of your apartment is one way to choose an exclusive agent.  For more suggestions about choosing an agent, go to Choosing an Agent.