GuessParity
From Avisynth wiki
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)
}