The phrase "fair guess" means making an educated or reasonable estimate or prediction based on the information available. It means giving a thoughtful and unbiased estimate or assumption. Full definition
BUT, it is an equally fair guess to say that one or the other has simply always been there from the beginning. (religion.blogs.cnn.com)
It's so funny thought — I'm a bit fairer I guess because I actually have to apply with a light hand or it is too pigmented on me! (beautynerdbynight.com)
$ 400 Aud would be fair I guess seeing how it's a brand new console. (n4g.com)