Once you've finished processing in Df96, pull a small section of the roll off of the reel and inspect it. Fully fixed film will have a transparent film base that can clearly be seen through when holding up to a light source (pinkish/purple dyes may be present with some film stocks). If the film base appears hazy and the emulsion side of the film exhibits a slight brown tint to it, the film is likely not fully fixed.
Df96 is a self-completing process and thus cannot over-develop or over-fix your film. If after the calculated processing time your film exhibits the above signs of under-fixing, simply place the film back in the developing tank and agitate for a bit longer until the film base has completely cleared. This will not damage your film at all.
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