Plato had his characters engage in a good deal of speculation, but generally was very conscientious about explicitly identifying it as such. Larger speculations are often explicitly couched as myth or poetic invention. All such things are explicitly considered no more than “likely stories”. On a smaller scale, verbal cues generally abound to tell us when things are intended in a more tentative way.
Plato and Aristotle were generally — each in their own way — extraordinarily good at this sort of thing. However, the much more “dogmatic” style of the Stoic school set a new default tone for the later tradition, all the way to the time of Kant. It became standard to present what was actually speculation as if it were a simple report on the truth, or a certainty grounded in a strong kind of knowledge. (See also The Epistemic Modesty of Plato and Aristotle; Kantian Discipline.)