Unfortunately, and as it happens for most jobs, there is no such a thing, or at least, not as immediate and clear as a grade of an exam.
While one can argue that the number of publications, and the quality of the journals\conferences you publish at, can be good ways to measure your worth, this can often be biased by many factors.
If you do not work completely alone, publications are often the result of team work. There are cases where it is hard to see who worked the most, or who gave the key contribution that made the paper be accepted. I have seen people with many published articles move to another group and loose productivity all together. This probably because the previous group supported the PhD way too much and they did not learn from their colleagues. I have also seen the opposite.
Despite everything, publishing papers to journals or conferences (even the best ones) can really feel like a lottery. Usually, when you study a lot for an exam, you are almost certain that you will get a good mark, this is not true for publications on any level. A paper can be hardly rejected by conference A and, instead, accepted by conference B. This can be because of many factors. Maybe conference A was not the suited one, or reviewers just did not like the paper despite its contributions, in any case, as you can see you cannot really value your work on this type of outcome. You can, for sure, improve from reviews, but I would never use it as a way to evaluate yourself.
Depending on the specific topic of your PhD, there are different standards expected regarding the number of publications you should have. Maybe, you just need a couple and, in that case, it is pretty hard to motivate yourself for years with just two inputs (two accepted papers). Moreover, even the best PhD students have to deal with rejected articles, sometimes, even more than they have to deal with accepted ones.
I would also like to add that PhD students are pretty prone to mental illness such as depression, anxiety disorders and so on . In my modest opinion (as an academic), these problems are also caused by the fact that many doctorate students evaluate their worth depending on this hard numbers (number of publications, number of paper not accepted).
Overall, I think that self motivation is a very personal thing. It is up to you to decide what motivates you and what makes you worth.
 http://www.sciencemag.org/careers/2018/03/graduate-students-need-more-mental-health-support-new-study-highlights (one of the many)