What is PhD research training? Are there any good indicators you might be a good researcher, without yet a full background in the discipline? What what be som

What is PhD research training? Are there any good indicators you might be a good researcher, without yet a full background in the discipline? What what be some of those qualities needed and sought for takin in my a professor. For example, say you just have an undergraduate degree in CS, what might be some ways to show you would be a good candidate without yet having written any publications.