QTGMC frame rate question

[Selur]

.. is there a way to tell whether the source video is tff or bff?

Sure, read: [INFO] Interlacing patterns.

What you would basically do, is:
a. overwrite the scan type to bff or tff
b. enable QTGMC in Bob mode
c. look at the output in the Vapoursynth Preview.
When you can move through the frame and the motion is fluid (in a scene with horizontal motion), the frame type you selected is correct, if you see forward and backward movement, you need to overwrite to the other scan type.

Cu Selur