How can we conclude that something is unarisen and eternal?

Let us suppose there is something (i.e. nirvana) unarisen and eternal. How can we conclude and trust that it is indeed unarisen and eternal ? Because in order to find out whether it is unarisen, we will have to go into its infinite past; and to find out whether it is eternal, we will have to go into its infinite future. Nirvana is said to unarisen and eternal. So my question is : How do we know something is unarisen and eternal ?