By checking all dimensions of the footprint against the datasheet values.
I found freecad + kicad-stepup + drawing dimensions is a good tool for this task.
(The newest version of stepup is only available via the freecad 0.17 addon manager)
This allows you to create similar drawings then i did in this pull request comment: https://github.com/KiCad/kicad-footprints/pull/60#issuecomment-345468460
Here a sketch of the footprint compared to the maximum pin dimensions: (I know i should have used some sort of statistical calculations to get the “true” toe, heel and side measurements.)