Should you look at girls boobs?

No and it is rude. Do you want girls to check out your body parts? Men who do this are rude and crude.

If a girl has covered her boobs and her cleavage then she does not want anyone to look at them. On the other hand if she has a cleavage showing, or scanty or revealing top, then she wants her boobs to attract the attention of any guys who she feels attracted to. Unfortunately this process is not so selective and very likely ALL guys will find her boobs attractive and will look at them. The girl will like this if she finds the guy attractive herself, but if she finds the guy repulsive, she will consider him to be rude. Unfortunately the guy cannot tell in advance whether the girl will consider him attractive or repulsive and rude.

You can however, avoid being considered rude by just having a quick glance at her boobs and watch for her body language. If she puts her arms back and kind of displays them more so, then she finds you attractive, but if she turns away or covers them, she does not want you to observe them any more.

This does not seem very fair on the guys - right?

I think there was a song one where the lines were "you can look at the menu, but you just can't eat; you can feel the cushions, but you can't have a seat". That's life.

In some states they have like a 7 second rule. If you look for more than 7 seconds it is called leering and this is a breach of the law, but less then 7 seconds is just normal visual attention to what is going on.

Obviously we cannot have a situation where a guy is breaking the law if he momentarily look at a girls boobs for a second or two. This would result in most guys having to wear dark glasses all day every day to avoid breaking the law. It would cause car accidents etc.