If a water-cooled engine REALLY overheats, the coolant will BOIL and at some pressure (set by the pressure cap on tank or radiator) the pressure cap will lift and CLOUDS OF STEAM AND BOILING WATER will be BLASTED out.
If this does not happen, it is probably OK.
You must remember it is a pressurised system, and it will build up pressure in normal operation, so if it is HOT and you undo the cap it WILL blow hot water at you, for sure, even in normal circumstances. Read the warnings on the cap, the user's manual, the Haynes Manual, etcetera.
The extra pressure stops the coolant from boiling till the temperature has gone well over 100 degrees C, and it could reach 110 degrees or more, which is VERY hot, and if you release the pressure by undoing the cap the coolant water-steam mix will practically EXPLODE out of the tank or radiator right into your face.
A good trick is this ...
The cap fits on "bayonet lugs". You push it down, turn to the right just a CLICK which stops it coming off, and then turn it about a quarter turn more. It's this last bit that sets the pressurising feature.
Fit the cap tight as normal, then without pushing down on it, turn it anti-clockwise till the cap catches the lugs and stops turning. It will not pressurise, and so it will boil sooner than normal - if it boils at all.
Now take it for a decent drive to heat it right up.
The coolant will not pressurise, so it will BOIL when it gets to 100 degrees, or a tad more if there's a lot of anti-freeze in it.
Try this and with any luck it isn't really "boiling" at all!