First thing you should do is begin looking for a broker who services the area you are looking to purchase in, a broker who specializes in a specific area will generally have the best idea of what properties are worth and be able to compare apples to apples. There is no sense in using a broker who works primarily downtown, say in Chelsea, who rarely even ventures to Harlem where you are looking to purchase. The next thing you want to do is go to their company's website and see their current and previous transactions, if they have a physical store front swing by their office or at least call the broker or a couple of their agents and get a feel for them on the phone. You can ask them to provide references too.
I specialize mainly in the UWS and even though its close to Harlem I still don't have the best knowledge of the area and its best investment potentials. What I do know well is other brokers who work and specialize in Harlem; Klara Madlin Owner and Principal Broker of Klara Madlin Real Estate (who already commented below) is an excellent choice and has years of experience in Harlem, and Robb Pair Owner and Principal Broker of Harlem Lofts, his firm IS Harlem and has extensive knowledge and experience in the area.
Klara Madlin Real Estate
212-580-3180 ext: 200
You could certainly start with either of those, they or one of their agents will be able to help you find what you're looking for.
Zhann K. Jochinke
Associate Broker - Argo Residential
Vice President - Manhattan MLS, Inc
Director - Manhattan Association of Realtors
(212) 896-8638 | email@example.com