Im currently working on an '02 sentra that had overheated. Diagnosis pointed to a faulty thermostat. The operation went smooth, but when it was time to fire it up, it struggled a lot and smoked lika chimney when it finally started. The cooling system is still not working properly and after couple days of headaches(still won't start) I noticed that the engine gets flooded when cranking, and the two middle cylinders were up to the plug hole with coolant. So before I go rippin' apart pretty much the whole engine, I wanted to make sure that it is in fact a cooked Head gasket and not a valve cover gasket,or something else. Also the main reason I don't want to disassemble much is because I don't want to have to replace gaskets and seals on everything I remove not to mention dealing with the timing chain. Is it the head gasket? Is that my only option or is there an easier way to do this?

Head gasket. 101 I'm an old motor head and the biggest design flaws was a steel block with aluminum heads. The two metals in time blow head gaskets a lot because the two different metals expand at different rates.
Basic checks for overheating
Check radiator full and not clogged/ clutch fan or electric fan good./ Thermostat good. Temperature sending unit is accurate. Timing is correct. Can see water start to circulate in radiator when thermostat opens after warms up water pump good.
All symptoms for Head gasket problems.
Overheating when sitting just by running
When idling shakes and pops like bad gas or timing
Runs bad no power up hill and pop (not ping) through carb/ exhaust.
Coolant on oil dipstick or the oil level is all of a sudden way to high and bubbles showing on stick smell dipstick for coolant smell.
Water steam out of exhaust pipe (not smoke) when engine and exhaust is already warm and hot. Put a rag over HOT exhaust and gets wet.
Coolant showing on intake manifold
Coolant showing around block spark plug.
Coolant dripping off back of motor (firewall) on ground ( which is a common area) and any dripping off head gasket seal to block.
Coolant in cylinders after spark plug is removed.
A compression check could show a bad head gasket but it can also show a bad valve seat or a bad compression ring on piston
The coolant/ two middle cylinders was a dead give away. YES a cracked head or a bad head gasket. Or both.
Reasons a head/gasket goes. Motor was overheated real bad warps head blow gasket. Bad head to start. Improper head gasket/ torqued head bolts wrong pattern or wrong ft. lb. or put in wrong or defect in head and cracked.
The valves has nothing to do with the coolant other than the head ports for coolant go through the heads to cool head. Here what you need to do. Take head off. Take to machine shop and have them pressure test head for small crack in head. If OK then have them surface the bottom of the head to take any warping out and flatten. Now some heads require new bolts to put head back on and ASK machine shop they forget to tell you. When you pull the head its pretty easy to see when looking at the gasket the blow by in the gasket from a water port if that was its failure point. Oh and one more thing if a piece of head gasket got into a cylinder and scored that 'Maybe throw car away or a new rebuilt motor"! If that needs honed too much now its pull motor and piston with bigger rings. Like I said and old motor head. From what I'm reading I feel you are going to be way over your head in this repair get a friend mechanic. I can almost tell you a garage repair is way too much and not worth the car. Again If the plugs are pulled and see coolant %100 head gasket blown. Watch putting on a new head with new valves/ shims/springs The motor being old the stronger valve job actually can suck the oil out of the crank. Your machine shop will tell you the same thing if its high mileage the motor.I realized I typed a lot of information here but I felt you needed it all and wanted to pass my rrrGGHHH of 35 years of turning wrenches

Signed 21 boat
If my answer helped rate this answer