If most of the inputs on a form are required, indicate the few that are optional. If most of the inputs on a form are optional, indicate the few that are required. When indicating what form fields are either required or optional, text is the most clear. However, the * symbol is relatively well understood to mean required.
Some people will argue that a site’s Web forms should be consistent throughout, and therefore every required input field should be indicated as such, regardless of whether or not there are any optional input fields on the form. While this may be useful in forms that require several pages to complete, the variety of Web forms people encounter online makes it hard to imagine that they will remember a specific site’s system of indicating required input fields across the one to two forms they encounter.