1996 Nissan Maxima Head gasket.

There are a number of tests that can be performed but most of them are not totally reliable. The only reliable test I can say is to sniff the fumes at the radiator cap using an exhaust analyzer that will tel you if there are any hydrocarbons from combustion in the cooling system.

You can do a blue fluid test at the radiator or look for white steam from the exhaust but these symptoms usually only show up in extreme cases where you were already pretty confident you had a blown head gasket.

There is another trick that can be tried too. Remove the radiator cap and tightly attach a latex glove to the radiator opening and run the engine for a while. If combustion gases are being pumped into the system, the glove will become inflated in most cases.

The bottom line is that finding a blown head gasket is not always black and white.

There is no such this as "Mechanic in a Can". While some of those sealers may temporarily slow down a head gasket leak, at the same time they can plug up other components like the heater core and radiator. I don't recommend them at all, especially if you plan to continue driving the car.