Common sense
Even if you dont have a law degree, or a background in criminal justice, common sense tells you that it's unlikely somebody will still be breathing after their airway has been restricted for over 8 minutes. If you've been on a guys neck for a long time, you dont tend to forget his airway has been restricted by a lot of your body weight for a long time, especially not when he's reminding you by saying he can't breathe. 8 minutes is not 2 or 3 minutes, enough time has passed by 8 minutes that adrenaline has stabilized some, especially with an alleged suspect in handcuffs the entire time, and other officers there to assist you in keeping the alleged suspect restrained. The question you would think a family pressing for first degree murder would ask is not whether the death was intentional (since theres not really a way to unintentionally kneel on someone's neck for 8 minutes, 2 or 3 minutes or so might be unintentional if the alleged suspect still dies), but whether any of the actions of what came before the arrest were premeditated, such as the alleged counterfeiting call and what events lead up to the arrest.
First degree murder is premeditated, that's simple enough, not all murder is first degree. Running over an animal on the road is usually an example of unintentional death, it's an accident and usually accidents happen quickly (though there are people who would run one over and claim it was an accident, most of the time it is an accident). Spending 8 minutes taking a life isn't something you usually do by mistake though, that's all common sense.