What words indicate that you think something is true, but you are not completely sure about it?

think, think it is possible, believe, bet, would gamble, would wager, guess, reckon, speculate, surmise, suspect, get the impression, have the impression, be under the impression, get the idea, feel, have the feeling, get the feeling,

What do you say when you think something is true, but you are not sure?

I think so, I guess so, as far as I know, to the best of my knowledge, as far as I'm aware,