One sure way to tell... Firewall the brake pedal and if the wheels don't slide, you are under braked... and overloaded.
I know that sounds extreme but. Lets imagine Granny Zog jaywalks in front of you and you have to panic stop to keep from making her part of your grill.
If you cannot stop in time and wipe out granny, chances are, you'll get charged with assured clear distance and possibly vehicular manslaughter and when the attorneys get don, you'll loose everything, all because you couldn't stop in time...why..
I say panic stop your brakes and make sure you can stop if the need arises.
Driving is like Russian Roulette. The above may never happen to you, but if it does, at least be prepared. PI and/or wrongful death is always expensive for the at fault party.