How can I distinguish between the three stages of Interphase (G1, S, and G2) ?

(G1) - The stage where the cell spends most of it's life. Doing it's job (job being what the cell was created for, such as being a muscle or bone cell.) and being alive.

(S) - S phase is where cell performs synthesis (Hence the name of the stage, S.) and duplicates it's nutrients and everything inside it, most importantly though you must know that this is the stage where DNA is copied.

(G2) - This is the stage where the cell is preparing to entire mitosis, it's also known as the pre-mitotic phase. This phase takes place immediately after DNA has been replicated in the S phase. After the G2 phase is finished, mitosis begins.