Underwriting a multifamily property can be very complex, and you must know what you're looking for. The first place to analyze would be the local market in which that property resides. You'll look at local market data such as the vacancy average, number of units to come online, population size, population migration, top employers, school ratings, and much more.
Then you can focus on the subject property and underwrite the properties existing numbers to validate their accuracy. That includes items like the existing vacancies, collections averages, current market rents vs. market rents, how much you'll need to make improvements, and much more!