I recently saw an apartment in a building which was converted from a rental building to a condo building in 2005. The seller's broker gave me a number for the taxes if there was no tax abatement, which number seems too low to me. Other than contacting the managing agent for the building, how can I check what the tax amount for the apartment would be without the tax abatement?

acris lists taxes, the offering plan would have info on the abatement.

I wonder about the methodology used also. For similar market priced apartments I found that the abated taxes at various new developments seemed quite different, and no one could explain that when I asked.

I've got a suggestion which may be a bit over the top & expensive: Contact a real estate tax certiorari attorney because they have an indepth understanding of NYC RE tax abatements. One firm that come to mind is Goldberg Weprin @ 1501 Bway. In any case, I would not rely on the broker for this.

Alternatively, just plan on selling before the next average buyer figure it out.

Find the building on propertyshark and in one of the columns you'll see how much the unabated taxes are for the entire building.
Then take the percentage of ownership your individual unit has (available in the condo declaration on acris) and that percentage of ownership is also your percentage of tax liabilty for the building.
Of course 5 years from now when the abatement is up, that number will likely be higher.

Lobster, I tried to do this too last summer when looking more seriously at Greenpoint/Williamsburg. It's a bit of an art. Start on the NYC property taxes listing site and see if you can figure out what kinds of comparable buildings would be used to calculate the taxes.
http://www.nyc.gov/html/dof/html/property/property_condo_coop_comp_rental.shtml

I tried figuring out worst-case scenarios for several buildings and concluded that a lot of the expected tax hikes espoused on the boards were over-the-top. Try some of the methods here and you can probably come up with a ballpark range.

This is actually fairly easy if you know where to look and have the specific apartment address. Use this site and open up a pdf of the quarterly account statement. If you scroll to the bottom, you can see exactly what the unabated taxes are:

http://nycprop.nyc.gov/nycproperty/nynav/jsp/selectbbl.jsp

I am very concerned about keeping my monthly maintenance expenses at a reasonable rate and therefore I want to be certain of the true amount of property taxes I will need to pay. Thanks to everyone for their very helpful contributions to this discussion and buyerbuyer, I hope that this discussion helped you as well.