How can the amount of DNA present in a cell give an indication of where in the cell cycle a particular cell may be?

During S phase the amount of DNA in the cell doubles since chromosomes are duplicated. As a result, the amount of DNA found in a cell in G2 is significantly higher than that of a cell in G1. If you know the amount of DNA that a normal cell has and find that a particular cell has much higher levels of DNA, the cell has likely already progressed through G1 and S phase, but not undergone division in M phase yet.