Good theology is what you can sell to the believers.
Bad theology is what does not sell. Actually this is wrong. Yes, OK, so according to atheists there is no objective reality which theology describes (and as a non-theist myself, I am inclined to agree). But some theology is definitely far more harmful than other theology, and some theology can be beneficial.
For instance, the kind of theology that says that God dislikes LGBT people is obviously harmful; whereas the kind of theology that says God is Nature and Nature is God, or that God is Love and Love is God (and not a person) is harmless, and probably beneficial as it encourages ecological awareness; and the kind of theology that encourages people to be nice to everyone because "we are all made in the image of God" is surely beneficial. Also the kind of theology that says we don't know if God exists or what She wants (e.g. Godless Morality by Richard Holloway) seems like a good thing to me, as it encourages people to be more humble and tolerant of others' beliefs.
Theology can be tested empirically by its effects on people's lives. Bad theology will make people unhappy and cause harm; good theology will make people happy and altruistic.
NB in my book, good theology includes old-style atheism (i.e. the kind that was more tolerant of other views), agnosticism, naturalism, pantheism, panentheism, deism, universalism, unitarianism, a lot of mysticism, and the interfaith movement. Most of these theologies recognise that mythology is a metaphor for the process of living, and is best enjoyed in the mode of fairy-tale. They also show respect for other philosophies and religions.