ICE is right. She shows the cleavage so you will look. It's all part of the mating game. A woman will consider it a compliment if you glance at her cleavage, then her face and smile.
However, consider the woman's maturity level to determine how she will respond to staring or ogling.
Older women who are comfortable with their sexuality won't be as offended by your staring because they've been around long enough to know that it's normal.
That being said, DO NOT look at the cleavage while a woman is talking to you. Look at her eyes.
If guys looked at the eyes more often they might be better at figuring out when a woman is lying to them!