Can you judge someone by the clothing he or she wears? Even if you can, should you?
The problem is that the term "judge" is one open to different meanings and often has a negative connotation.
The truth is that you can certainly make inferences about people based on their choice of clothing. Some inferences are more accurate than others.
I saw a couple of gentlemen on Friday, at least one had tzitzis and at least one had a yarmlke (under a cap). It is a reasonable inference that these gentlemen were Jewish but they were both self professed non-Jews (Messianic though). The inference that they were Jewish would be technically incorrect but they clearly wore their clothing out of an affinity towards Jewish belief and practice (albeit incorrect in major aspects) so it is not totally without basis.
If you see someone dressed not in accordance with halachah, this speaks to their fidelity to those halachos (though we must be dan lkaf zchus when possible).
People present themselves how they want to be perceived, and if they don't care then that will typically be made apparent also. You can not always tell the authenticity of the presentation, but the fact that such is how they want people to perceive them in and of itself tells us certain things about them.
Of course you can.
If a man wears only white shirts and black pants, he is likely trying to fit into the 'yeshivish' crowd. If he is wearing a large knit kippah and a colored shirt (or even a non-button-down shirt), he is likely trying to fit a more modern type. Likewise, with women and their styles of dress.
To be honest, it is very easy to judge people based on their clothing. Most of the time, these judgments are accurate.
The real question, then, is whether you should be doing such judging.