unobvious, unproveable, untestable, undisprovable, indemonstrable, nonprovable, improvable, undemonstratable, unverifiable, unproven, more...

provable, disprovable

impossible to prove,   difficult to prove,   unable to be proved,   cannot be proven,   not able to prove,   unable to be proven,   unable to prove,   not testifiable,   unable to be shown