To hear her speak, I would think she is from Tokyo.
I can't imagine a situation very well when this sentence makes sense.
Could it be used like this:
A: ~. So my daughter have gotten very much used to Tokyo dialect.
B: To hear her speak, I would think she is from Tokyo.
I think it's forced. Could I request a convincing example?
I think the original sentence should have been "you would" rather than "I would."
For example, "A new student has joined us. To hear her speak, you would think she is from Tokyo."