Well, you'll know if you have air in your brake system if you push your brake pedal and you don't feel your brakes working. Also, a pulsing braking system (not like ABS turning on) can be from air in the system. I think you'll know clearly when you have air in the system. But unfortunately, you may not find out until it's too late and cause an accident. You can try rebleeding the brakes with new fluid just to make sure.