Can one determine Enneagram type through astrological birth chart analysis alone? While the natal chart can provide potent clues as to type, I do not think it can be used to "diagnose" type at this time--nor is that necessarily desireable. After all, the introspection required in discerning type is more important as a process for personal growth, than is coming up with an "answer" per se. Determining Type is not the goal of the work, but simply a potent tool for the real work: Liberating the individual from compulsive reactions, so that one may live out of the True Self, or "Essence". The work is profoundly spiritual.
Traditionally, discerning Enneagram type has been a process guided by someone well versed in the system as a whole. It is only recently that respected Enneagram teachers have offered self-assesssment tools and questionnaires (some available through the links on this site) for the seeker.
I am still very cautious, even reluctant, to recommend such tools without the additional insight of a teacher trained in type discernment. This is more true for the Enneagram than for any other personality system, for the Enneagram is designed to identify material that gets beyond the idealized self-image, and to expose the unconscious (and unflattering) core motivations of the personality. Self assessment tools are ultimately only as effective as the ability of the user to correctly identify their own unconscious drives and motivations. It requires self awareness and humility.
Both Enneagram and astrology teach type through lists of traits or characteristics belonging to a given type or sign. In my own theory of Enneastrology, I note specific chart factors that are correlated with a given type. For example, a Moon in hard aspect to Saturn is likely to show the personality traits of Ennea Type Two. A Mars that is completely unaspected or otherwise isolated in the birth chart signifies traits of Type One. But what if a birth chart holds both those chart factors? Is the individual a type Two or a One? Indeed it is common for a chart to hold factors delineating more than one type. Clearly more detective work is needed to determine how these chart factors are actually experienced and lived out, in order to discern which one is the core type. Always, dilaogue is the preferred mode of operation.
It would be nice if type could be determined simply through astrological chart analysis. Maybe some day it will be possible. Nevertheless it is my hope that Enneastrology will contribute toward a deepened appreciation of both traditional Enneagram and Astrology teachings, and what these two great traditions have to offer one another.