With the same computer, same OS, and same version of ffmpeg and decoder, does ffmpeg always produce the same YUV file when decode a video file? By same I mean the YUV value is exactly the same.
A search didn't produce any indication of non-deterministic video decoding for a
digital --> digital signal flow.
In any case, you can first run :
ffmpeg -i in.mp4 -flags +bitexact out1.y4m ffmpeg -i in.mp4 -flags +bitexact out2.y4m
And then run
ffmpeg -i out1.y4m -v 0 -f md5 - ffmpeg -i out2.y4m -v 0 -f md5 -
Compare the MD5 values. Same here for H.264 input.