Pick up a copy of Buenos Aires Tango magazine when you arrive. It's available at milongas, classes and stores that sell tango clothes. You can also find it at some hotels. All the milongas and other tango events will be listed.
Find out the character of each milonga you are considering. For example, some are traditional, some attract older dancers, some are alternative and some are held outdoors. The Internet and tango guides in Buenos Aires may have some information about the milongas, but the best approach is to talk to local dancers.
Take local tango classes. Classes designed for local students and taught by local dancers will introduce you to the Buenos Aires tango community. When you see someone whose dance style you especially like, whether teacher or student, ask her which milongas she enjoys most and find out why she enjoys them.
Attend local practicas. At practicas, you will have a chance to dance with the locals in an informal setting. When you find a strong dance connection with another dancer, ask him where he will be dancing that evening. Not only will you find a milonga to attend, he will also probably ask you to dance with him there.
Socialize at milongas. Ask other dancers where they are going later that night or where they will be the next night. You may find out about milongas that you wouldn't otherwise attend.
