Unhooked battery now rad cooling fan wont come on

check the rad and cooling fan on motor also check fan relays
make sure rad colling fins are clean if all this check out o.k.
cylinder heads may be plugged up try reverse flush if it dont help
head gaskets or heads

You can buy a flush kit from your local auto supply store to add to your cooling system for this and future flushing. As for changing your thermostat - simply remove the thermostat housing - remove the old thermostat - replace with new (be sure it is the one rated for the season at hand) - replace gaskets (or use RTV gasket maker) and reinstall housing. Fill the system with proper % ratio of coolant to water (usually 50/50). Drive away happy.:) PS You can find your thermostat housing by following the top rad hose from the rad back to it at the intake manifold.

If you have white smoke from the tailpipe, coolant in the engine oil, engine oil in the cooling system, that indicates a head gasket failure. However, the most definitive way to test is to have a hydrocarbon test done on the cooling system (checks for exhaust gasses that obviously don't belong in there).
If the system comes up clean, you still can have either a leaking intake gasket, partially obstructed radiator (many times flush will go around obstructions not through them), a malfunctioning electric fan or fan relay, or even an obstruction in front of the a/c condenser or between the condenser and radiator (leaves, plastic bags etc) Don't forget though even if the cause is other than a gasket, if repeatedly or severely overheated the head gaskets can also fail. That is why complete testing is very important.
good luck

is the electric cooling fan operating properly.

Thermostat, cooling fans, water pump, rad (not plugged wirth scale)

Usually near the thermostat.
By the way, remove the thermostat and drive around with the car. If it still overheats, might have some other problem, like you said, temp sensor, or some kind of electrical problem.

Flush rad & change coolant & thermostat. Check operation of rad fan, & make sure it is coming on when engine reaches normal operating temperature. Be sure to bleed air out good when refilling rad/cooling system. Start engine & turn heater on when refilling & let rad fan come on & off at least 2 or 3 times before putting cap back on rad, and refill rad as needed as you do this. then fill return bottle to proper level & see what happens.

Remove rad cap when cool, top up coolant to 1 or 2 from top. Start engine, with heater on (not defrost) and let it run. When up to normal operating temperature, watch for good flow in rad, which tells you thermostat has opened. Fan should come on shortly after you see that flow. I usually give it about 3 times with fan on & off, before I put cap back on. Then top up return bottle to proper warm/hot level. Monitor gauge too, just in case gets too hot for some unknown reason. If gets too hot, shut it down & let it cool, and retry. I usually rev it once in a while for 5 seconds or so,( just increase a little) just to add a little more flow to help push out air.

make sure engine is cool then drain coolant by opening the drain plug at bottom of radiator. remove cap from radiator so coolant will continue to drain and when coolant is down enough to pull off upper radiator hose at intake then close drain, then remove two bolts at coolant housing and remove thermostate. clean gasket material from intake and housing and replace with new gasket and thermostate tighten bolts install hose fill rad with coolant water mix 50/50 and run engine with heat on floor until gets hot and you see coolant flowing in rad, install cap

Check for a switch at lower part of rad. Should see a wire harness & plug on a sensor at bottom of rad. This sensor/switch also operates fan on/off according to coolant temperature. You may need to disconnect battery to get at it safely.