Only way would be to collect a few quotes and move forward with the one that provided the best set of terms. Generally most people you speak with are going to provide you a quote with their "standard" commission built in and anything less would be them taking a reduced commission to win your business.
As standard practice I offer my best pricing the first time to not have to go back and fourth like a car salesman to win business. However I have made exceptions in the past for certain clients.
Something to consider though, just because someone is offering a better rate does not guarantee the loan will close or that it wont be filled with headaches. DSCR loans have MANY variables that can impact eligibility and those variables all vary from lender to lender. Understanding those variables is important in making sure the deal actually closes. A new loan officer or one not familiar with DSCR might offer you the best interest rate and fees on a DSCR loan because their pricing engine told them, but your specific scenario may not fit.
for example, a deal I am currently working, First time investor, First time home buyer, living rent free, seeking 80% LTV purchase, Using Short term rental income to meet DSCR, with gift funds.
Each of those things individually can be done at one lender or another, but knowing if it can be done at a single lender is the difference between someone who knows what they are doing vs someone just quoting interest rates.