Of course, one cannot say if it's 100% true or 0%.
However, between the 2, there are possibilities.
If you're above 50% of probability, you can say, you tend to believe it. If it's below 50%, you can say you tend not to believe it. For me it's above, so I say, "i believe it", which doesn't mean I'm sure of it.
I don't think anyone is precisely on 50%, though I suppose one can be some days above and some days below.