FFDShow and IDCT

[Waleska]

Fragment from the official ffdshow wiki: "If libavcodec's auto detection fails for some reason, the proper IDCT can be selected here manually"

So let's suppose I have IDCT set to "Auto" and ffdshow fails to match the correct IDCT, how can I tell it actually happened without analyzing/comparing screenshots? what does it fall back to?

Thanks.