@Karolina Powell
A lot of people have mentioned different aspects of this in their posts. For rental income, you can tie it to leases, which are legal documents. Therefore, the leases should be accurate. You can also ask the tenant to sign an estoppel, which essentially has them verify the rent they are paying.
As for expenses, the main areas where they can go awry include property taxes, which you can determine on your own; the mortgage, which you can also calculate; insurance, for which you’ll have a quote beforehand; and utilities, for which you can obtain the bills from the landlord. Ultimately, the most variable expenses are discretionary ones, such as repairs and maintenance. During your inspections, you can assess these expenses yourself.
It's also important to step back and evaluate expense ratios to determine if they make sense.
For smaller multifamily properties that are usually listed on the MLS, I would find a MLS agent who owns investment properties themselves, as they'll better understand the process. Many agents claim they can handle multifamily, but working with one who owns similar properties provides better confidence that they know what they are working with.
However, if you are looking for larger multifamily properties—which are not listed on the MLS—it’s essential to build relationships with those brokers, as they often don’t list these properties on central services. I would also recommend checking websites like LoopNet.