graduate admissions - How can anyone assess their own Phd application prior to actually applying to determine its weaknesses and figure out what to do next?

How can an undergraduate student self-assess his/her PhD application prior to actually applying for PhD programs?

As prospective PhD students, we can generally compare ourselves to other students at our own school using our grades. We may also apply to internships or awards and, by acceptance rates of such programs, we may begin to understand how "good" we are as a possible Phd student candidate. In addition, we might try discussing our potential application with a professor, but even though we might be really good, the professor might be too busy or otherwise not give great feedback. In such a case, how can one self-assess himself/herself on how good is his s/he perceived as a prospective Phd student, determine his/her weaknesses, and figure out what to do next ?