well heat is fast moving particles they collide with slow particles which is cold and the slow particles move fast too. In the end the slow particles move faster just as heat so they are not cold...
When it begins to simmer/boil/bubble, it's probably hot. Take a spoon and put it in the sauce. Remove the spoon and insert it in your mouth. Is it hot? If so, the sauce is hot. If not, it's not....