How can you determine that a star is about to turn into a red giant?

When the inner core shrinks and the outer atmosphere expands thereby giving red color to the star.

Explanation:

When the hydrogen with in the core of a  star gets exhausted, the nuclear reactions taking place in the inner core move outwards towards the atmosphere of the star in search of fuel. Due to this, the star expands and become cool and red. This process continues till the star convert into red giant and all the available fuel (hydrogen) is consumed.  

Due to expanding atmosphere,  the inner core starts shrinking due to gravity. The pressure and temperature also increases  in the middle till nuclear fission starts again but this time it will use helium instead of hydrogen.