To guess something, without having much or any proof. (Cambridge Dictionary)
A thought or idea based on scanty evidence: Conjecture. (Merriam – Webster)
To think or infer without certain or strong evidence; conjecture; guess. (Dictionary.com)
Suppose that something is true without having evidence to confirm it. (Oxford Dictionaries)
If you surmise that something is true, you guess it from the available evidence, although you do not know for certain. (Collins Dictionary)
Meaning of surmise. Wordpandit.
Mathematics, Substance and Surmise.