I don't know why people are giving you a hard time here. The quote is widely-known, and I appreciated your remark about the troubling lack of falsifiability criterion.
See elsewhere here, there's no shortage of experiments to be done that could be determinative in this case. Also, Karl Popper's rule is a great rule of thumb, it really is; but there's an immense bibliography in philosophy of science about whether it's a shoe that fits all circumstances.