Their effectiveness depends on their visibility and how directly the indicate the link. Persistant visibility wins over one based on condition and direct indication of interactability is more efficient than an indirect one. Tool - Example Visible Indication Comment Color Default link color in many browsers is blue. Limited use to color-blind people. Geometric appearance Underlines to links, 3D button effect, standard form widgets, etc. persistent direct Works for all seeing people.
Convention helps though.
It is obvious that some methods are more effective than others, and as it turns out, color and geometric appearance are the most effective ones . No other methods offer both persistency and direct indication. Combining two or more methods at a time to gives even stronger effect. Blue and underlined links that turn red on mouse-over, for example, are a very strong clue. Some might think that using all methods at once will give the best results.