@Mendy Blasberg I will assume that by multi-family you mean 5 or more units. The 2-4 unit building are still considered residential and are appraised using comps which tend to be harder to find. 5+ multi family on the other hand are appraised using their "economic" value which basically uses the NOI and cap-rate. The cap rate is determined by market, neighborhood and property type. CoStar publishes a report every year on cap rate for the major us markets.
I agree with @Jeff Louis you need work with a property manager who knows the market and can guide you on what these units can rent for. Are they rented at market? How much rehab is needed to bring them the rent at market? Is it worth it based on cap rate?
Another way to look at it is by determining your target return. and ask yourself what price should you pay for this property in order to get the return that I want. The interesting about commercial real estate is that they don't always have an asking price so you have to offer the seller what it is worth to you.