Gathering information on a potential investment

[Cristian Romero]

Hello everyone, 

I am looking to purchase a multifamily. I am having a hard time understanding how to calculate basic information like rent income. I have read a few books and they explain the formula behind the calculation for NOI and Cap rate, but they do not explain how to gather the information. I have hoping someone could explain to me how they dealt with this problem. I was thinking of knocking and just asking the tenants.