A script to guess the parity of your clip (thanks to mf).

function GuessParity(clip input)
{
 input = AssumeFrameBased(input)
 global guessparitytff = AssumeTFF(input).SeparateFields().Bob()
 global guessparitybff = AssumeBFF(input).SeparateFields().Bob()
 istff = input.AssumeTFF().Subtitle("Guessed TopFirst", x=20, y=20)
 isbff = input.AssumeBFF().Subtitle("Guessed BottomFirst", x=20, y=20)
 ConditionalFilter(guessparitytff, istff, isbff, "YDifferenceFromPrevious(guessparitytff) + YDifferenceToNext(guessparitytff)", 
   \ "<", "YDifferenceFromPrevious(guessparitybff) + YDifferenceToNext(guessparitybff)", false)
}