In German language you use an expression mit an Sicherheit grenzender Wahrscheinlichkeit to say that you are almost one hundred percent sure/certain.
A particularity of that expression is that it has a subtility that I think comes from probability/mathematics; you say that you are not one hundred percent sure, but you can come arbitrarily close to certainty.
The expression also can make someone smile and/or shows a little bit of sophistication in the usage of the German language despite it's somewhat confusing meaning.
A probably wrong but literal translation could be something like with probability bordering/verging on certainty.
I use the expressions almost certain, very sure, pretty sure, etc. in this case, but I wonder....
In English language there is an expression that can come very close to the German expression?