If one prays istikhara (استخاره) to determine if a partner is suitable for marriage, how would one determine whether the outcome is positive or negative?
Firstly, pay attention that you ought not to do Istikharah directly. It means that you should observe its steps at first. Actually in the first step you have to think regarding the issue and survey its aspects. (Afterwards, if you couldn't decide yet, then go to the second step)
Secondly, as the second step, you ought to consult confident and experienced people. Afterwards, if you still couldn’t be certain in your decision, then you can refer to Istikharah.
But in regard to Istikhatah that you inquired, you can refer to the holy Quran to do it. Of course you'd better to refer to the expert people (in Islamic fields) to do it for you. Anyhow, for instance, you specify your intention that would she be an appropriate case (wife, husband) for you. Accordingly, if the result in the Quran be positive(good), it indicates that she would be an appropriate case for you, otherwise you'd better ignore it.
In other word, Istikharah could be considered as a consultation by God (perhaps indirectly), afterwards Allah would inform you regarding the matter if would it be suitable for you or not.
Assalamu alaikum (Peace be onto you)
Based on my understanding of the meaning of the dua (prayer): [...] If marrying "so and so" is good for my religion, my family, my wealth, and the outcome of my deeds [...] then destine that person to me and make it easy for me. And if marrying that person is bad for my religions, my family, my wealth, and the outcome of my deeds [...] then keep us away. Destine whatever is best for me [...]
So, I hope it makes it clearer now.
NOTE: If anyone has the full dua in Arabic with the proper translation feel free to edit my answer as I don't speak Arabic and that was my basic understanding.