Yes, if you are a careful people, you will find that people with colored contacts look more beautiful but in an unnatural way. To judge if a person wear colored contact lenses or not, you need look carefully about their pupils in their eyes. People with colored contact lenses always have the same size of pupils no matter how bright or how dark their eyes are. Their pupils look too perfect to be real.
That just depends on the color of contact lenses you wear. Generally speaking, it is difficult to tell whether someone is wearing non-colored contact lenses. However, it is easy for people to tell whether you are wearing colored contact lenses because the color of the contact lenses is obvious if you wear colored contact lenses which make contrast with your original eye color. For example, if you wear blue contact lenses over your black eyes, it is easy to know you have worn colored contact lenses.