You can write to the NYPD to see what records they might have for someone on the force:

New York City Police Department
Personnel Resource Unit
1 Police Plaza Room 1014A
New York, NY 10018

I sent a request in for my great-grandfather's records at the end of March last year and by mid-July I received a nice cover letter, his "Force Index Card" and "Transfer and Assignment Card."

From those I learned some excellent information: His exact birth date, naturalization date, addresses, precincts, previous work before going on the force (he was a Hostler, or stableman), retirement date, death date, pension amount.

Fore reference, you can see an image here on my blog:
http://currach.johnjtierney.com/2010/10/michael-tierney-poli...

A cousin received much more info and even a photo of his great-grandfather.

If you send in a request I suggest you include as much information as you can - all I had was his name, a possible death year and his addresses from the 1900 and 1910 censuses.

I sent in a self-addressed, stamped envelope but they did not use mine and sent the info in a large envelope (postage was greater than $1.20 too.)

Good luck!