Bitt is correct. Tribune, the company that provides the data has a data element called "First Airing" which indicates whether or not the episode is "NEW." There is also another data element for "Rerun" however. And of course the "original air date" that Tony mentioned. There's also an episode ID data element which can be compared to existing recordings - as long as it's set correctly.

It's apparent that TiVo doesn't try and make any smart guesses about the validity of the values or lack of values.

TiVo's scheduling is super simple and they don't really do much in the way of advanced conflict resolution or fancy guess work. This has its advantages. KISS after all.