How can I be sure if my car has a blown head gasket?

The car is a 97 Lumina 3.1L V6. The previous owner said the car overheats. I properly bled the cooling system, and added almost 2 gallons of coolant. I used my head gasket tester and It took several minutes, and multiple pumps for the solution to change color. I'm just wondering if that is normal for this test, or if it's not the head gasket...