If the theory is well-formulated, rests on vetted facts, and you understand the context sufficiently, you can say: "This is a good theory." If it's not well-formulated or does not rest vetted facts, and your understanding is sufficent, you can say: "Bad theory."
There's an entire world of variation between those 2 extremes, but there's no reason implicitly why you can't understand a theory just by reading about it.
If the goal is to evaluate a theory's virtues, there's nothing stopping you if you have access to the correct documentation (provided it exists). Not sure what you think is missing.
If the theory is well-formulated, rests on vetted facts, and you understand the context sufficiently, you can say: "This is a good theory." If it's not well-formulated or does not rest vetted facts, and your understanding is sufficent, you can say: "Bad theory."
There's an entire world of variation between those 2 extremes, but there's no reason implicitly why you can't understand a theory just by reading about it.