Open houses are an excellent way to become familiar with the buildings and apartments in your price point in a particular neighborhood. Most are held on Sunday afternoons and you do not need to attend with a broker. (Note that not all buildings allow open houses, in which case showings may be done by appointment only.)
The sign-in sheet
Many buyers visit open houses on their own either while working with a broker (writing the broker’s name on the sign in sheet) or while getting their feet wet before choosing a broker. If you sign in without a broker, and want to make an offer on the apartment, you usually have the right to bring in your own broker at any time up until an offer is submitted, no matter what the seller’s broker (who now has to split the commission) tells you.
There are two important exceptions: If the seller’s broker is not a member of the Real Estate Board of New York (REBNY), which is more common in boroughs outside of Manhattan, they are not obligated to split their fee with another agent (a.k.a. "co-broke"). The same is true in some new developments where the developer's agents are in-house or not REBNY members.
In these two circumstances, although you have a legal right to choose an agent to represent you, you may have to pay them yourself.
How to "do" an open house
Walk through the apartment as if you live in it, thinking about your daily routine. Don't assume that you can change something, like expand the bathroom or add a washer-dryer. Could you live with it the way it is?
Don't take the broker's word for anything--from claims that there are offers on the apartment, to the cost of existing renovations or future ones, to the seller's reason for moving. Ask for documentation on issues like these--you probably won't get it, but the broker's reaction can tell you a lot. You may find that brokers let their guard down more toward the end of an open house.