Clothes can sometimes tell a lot about a person but it can be very dangerous to judge people based on their clothing. For example, I have had female friends that dress on the "sexy" side, giving the wrong impression to some men but were actually quite conservative and intelligent. I have also known people who refuse to purchase clothes from anywhere but discount stores (such as Walmart or K-Mart) and are very financially well off.
People dress the way they do for numerous reasons. Judging people based on their clothing can lead to misinterpretation.
I appreciate that Megan addressed the issue of stereotypes in previous posts. While certain types of clothing can indicate cultural issues or employment issues, that's the only real certainty. Sometimes people where black because they like black or it suits their mood. Sometimes people want to convey a certain image. It says no more about them as a person than does their skin color or economic status.
In some cases clothes can tell a lot about a person; however, one has to be careful to not stereotype people based on their clothing. A person's clothing can give you clues into the type of job they have (example, uniforms or business suits) or what they do for various activities (example, exercise clothing).
Clothes can sometimes tell us a lot about the person who wears them, but the signals can be confusing if you are not really familiar with the culture of the person wearing them.
Some clothing choices tell us fairly obvious things about a person. Where I live, if you see a woman wearing a head covering, it tells you she is probably a Hutterite. In cases like this, clothing can tell us something about the group to which the person belongs.
Sometimes, clothing can tell us something about the attitudes of the person involved. For example, a high school student wearing all black clothing may well be a "Goth." Such a person may be into the supernatural and things like that.
However, it is not always easy to tell what a person is like from their clothing. For example, you might think a person wearing worn out clothing is poor. However, in some places, this kind of clothing (even with holes in it) is considered to be fashionable. This means that it is sometimes dangerous to assume things about people from their manner of dress.