The first disciples knew him in the flesh in those days when he walked this earth. We might envy them for that, thinking it would have been easier to believe had we seen him with our eyes. You can know, if only you're willing to know. If you allow yourself to accept him as real, then you will know. Accepting something as reality is all knowledge is anyway.
All human knowledge involves at least a little bit of faith.
Of course, there is always the final cop-out. If Yeshua really is who they say he is, it is of supreme importance to know.