No way to tell until it's time to pull the fret.
Luthiers did it for decades without the application of heat. I know I did. But it's easier to pull the frets without chipping the fingerboard when using heat. And since most glues used to hold down frets (even super glue or epoxy) begins to break down around 180 degrees, might as well use a soldering iron to apply the heat directly to the fret. Good technique with the fret nippers helps, too. Slow & steady.