When enlarging you could use a substitute-film for focusing. There had been substitutes offered with special graphics (fine lines etc.) been exposed on a strip of 35mm film. After framing and roughly focusing your negative, it is substituted by that special one. At least when working with a two-glasses film cassette in your enlarger and orientating film and substitute the same way, that approach should facilitate critical focusing with difficult negatives.

Last year or so we had a lenghty discussion here about adjusting a grain-focuser.