I observe water vapor and water drops from the cooling water tank while driving in crowded streets and i think that the radiator fan doesn't start , how can i know if the fan is ok or not or that the

When water gets heated, pressure builds up in the radiator and water vapor is sent to the reserve tank.
And vapor dilutes into the water in the reserve tank.
When the engine cools down, in the radiator, vacuum builds up. To fill the vacuum water is pull from the reserve tank.
If vacuum leaks from radiator, gasket, etc., reserved tank won't able to function

NEXT STEP,HAVE A COMPLETE COOLING SYSTEM INSPECTION SERVICE PREFORMED AS FOLLOWS;
INSPECT COOLANT LEVEL,
TEST SPECIFIC GRAVITY,
PRESSURE TEST SYSTEM,
PRESSURE TEST RADIATOR/EXPANSION TANK CAP,
INSPECT DRIVE BELT(s),
INSPECT COOLANT HOSES,
INSPECT HEATER HOSES,
INSPECT BYPASS/MISC. COOLANT HOSES/PIPES,
TEST THERMOSTAT OPERATION,
INSPECT RADIATOR TEMPERATURE,
INSPECT RADIATOR COOLING FAN(s)/CLUTCH OPERATION,
INSPECT CONDENSER/AUXILARY COOLING FAN(s) OPERATION(s)
INSPECT RADIATOR & CONDENSER FOR DEBRI,
OBSERVE COOLANT TEMPERATURE GAUGE OPERATION,

First things first....
INSPECT COOLANT LEVEL
Low ????
PRESSURE TEST SYSTEM
PRESSURE TEST RADIATOR/EXPANSION TANK CAP
INSPECT DRIVE BELT(s)
INSPECT COOLANT HOSES
INSPECT HEATER HOSES
INSPECT BYPASS/MISC. COOLANT HOSES/PIPES
TEST THERMOSTAT OPERATION
INSPECT RADIATOR TEMPERATURE
INSPECT RADIATOR COOLING FAN(s)/CLUTCH OPERATION
INSPECT CONDENSER/AUXILARY COOLING FAN(s) OPERATION(s)
INSPECT RADIATOR & CONDENSER FOR DEBRI
OBSERVE COOLANT TEMPERATURE GAUGE OPERATION

If you need further help, reach me via phone at https://www.6ya.com/expert/ralph_2f0886b7d6af32ce

Begin with the basics....
INSPECT COOLANT LEVEL,
Low ????,PRESSURE TEST SYSTEM,
PRESSURE TEST RADIATOR/EXPANSION TANK CAP,
INSPECT DRIVE BELT(s),
INSPECT COOLANT HOSES,
INSPECT HEATER HOSES,
INSPECT BYPASS/MISC. COOLANT HOSES/PIPES,
TEST THERMOSTAT OPERATION,
INSPECT RADIATOR COOLING FAN(s)/CLUTCH OPERATION,
INSPECT CONDENSER/AUXILARY COOLING FAN(s) OPERATION(s)
INSPECT RADIATOR & CONDENSER FOR DEBRI,
OBSERVE COOLANT TEMPERATURE GAUGE OPERATION,

If you need further help, reach me via phone at https://www.6ya.com/expert/ralph_2f0886b7d6af32ce

First check to make sure you have no oil reidue in your cooling system. What you describe is a common symptom of a blown head gasket or other combustion chamber leak. Also check your oil and make sure it doesn't have a milky appearance. If both of those check out, run the car with the radiator cap off and let it get up to normal operating temperature. You should see the coolant flowing through the radiator. If you don't see the flow at normal operating temperature, suspect the thermostat. When the car reaches normal operating temperature, the fan should also cycle on and off. If it doesn't check your fuses and relays to make sure they're OK. If they're good, try applying a seperate 12V source to the fan. If it powers up then suspect the fan's thermostatic control. Good luck.

it is possible that when the water pressure rises in your radiator, and the radiator shoots the water into the reserve tank, that hose is not properly put on(happens alot when changing radiator) and the coolant leaks rapidly through there. another thing i suggest is observe your fan behavior. fans usually are only supposed to turn on for a couple of seconds, around 10 or 15. park your car, and let the car reach normal operating temperature. when the car starts to over heat, check to see if your fans turn off at all. if they do, then it is most likely the tw swith,or the water temperature switch. if you notice the fans stay running, then i suggest obesrving your hoses and replaing any that spill coolant because even a pressure test on your cooling system can miss a small pucture in a hose, which after a while will cause your coolant to leak, and your car to overheat. hope this helps. if still nothing, drop a comment and ill help you out somemore, but for now, this is the practical thing to do. hope this helps, goodluck.

It could be the fan temp. switch but it shouldn't have been boiling in the overflow tank. I'd recommend doing a compression test to see if you have a blown head gasket. If it's a blown head gasket, the compressed air/fuel mix being force into the cooling system could well look like boiling in the over flow tank.

have u bled the cooling system ?warm the car and cut the car off right before the fans come on ,let sit for 30-35 minutes,see if the coolant drops in res. if so repeat process untill coolant stops droping ,then drive car regularly if overheating stops ,but monitor it because it will drop somemore/there has been reports of new and rebuilt water pumps nt having correct propelar mounted on them/ backwards etc. if this helps let me know [email protected]

I think you mean 'radiator' and if you do, you probably have a leaking core or tank although it could be something as simple as a pressure cap that isn't screwed on correctly. 
Once it starts, you need to stick your head under the hood (open it carefully, you might get a face full of steam or hot water) and search for the source of the leak. 

Welcome to fixya

this could be cause by the thermastate or the fan could not be working i would replace the thermastate and check fan make sure it is running when the car heats up