Grandpa, a bit of a rascal, had genuine faith -- Dad, a "straight arrow," was, sadly,
                           I believe, a non-believer.
People can often tell what is real and what is not.
  Grandpa was not shy about cracking an off-color
                           joke at the dinner table, but he spoke admirably of Billy Graham, and their was a clear earnestness in his prayers and
                           his talk of faith.
In many particulars the "content of his faith" was evident,
                           though he was a rascal at times.
The following passage shows those "born after the flesh"
                           seem to know who is "born after spirit."
New Testament passage Galatians 4:
22 For it is written, that Abraham had two sons, the one by a bondmaid, the other
                           by a freewoman.
23 But he who was of the bondwoman was
                           born after the flesh; but he of the freewoman was by promise.
Skipping to verse 28 (full
                           text latter). 
28 Now we, brethren, as Isaac was, are the
                           children of promise.
29 But as then he that
                           was born after the flesh persecuted him that was born after the Spirit, even so it is now.
30 Nevertheless what saith the scripture? Cast out the bondwoman and her son: for
                           the son of the bondwoman shall not be heir with the son of the freewoman.
31 So
                           then, brethren, we are not children of the bondwoman, but of the free.