We believe it to be true because it is true, it is not true because we believe it to be so.
Perhaps this belongs in the philosophy discussion, but other than based on one’s own observation or experience, or the testimony (or instruction) of someone else, is there any other way to know anything?
Assuming, for the sake of discussion that all of Scripture is inspired and true, the only to know such is through the experience/testimony/teaching of those who have preceeded us who ***could ***testify based on personal experience.
Of course, if it isn’t true, the only way to know that would be through the testimony of others, as well.
So, to answer your OP, no. Other than through the teaching and traditions of the Church and her members, I’ve got no other reason to believe it to be true (except for the fact that it is true). That was kinda the idea that Jesus had when he commissioned the Apostles, wasn’t it?