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.

the oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. this blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. from the left ventricle, the blood passes through the aortic valve to the aorta.

deoxygenated blood leaves through the right ventricle through the pulmonary artery. from the right atrium, the blood is pumped through the tricuspid valve (or right atrioventricular valve), into the right ventricle.

before, trains worked using the steam engine. thus, it uses heat so it is crucial to apply heat balance. the steam engine absorbs energy during the stage of boiling water. so, this is the endothermic reaction. when the steam is used to work the piston, it has to release this energy through running the train. the excess energy is released as an exhaust to the chimney. this is the exothermic process because it releases energy to the surroundings.