Coal in principle consists of finite graphene sheets with all sorts of features. For stone coal these sheets are bigger than for brown coal and an extreme with respect to sheet size would be anthrazite. Coal also contains significant amounts of gas inclusions of up to almost 50%.

Criteria would be:

-XRD: mosaicity/crystallinity in general, relative orientation of the sheets, sheet size

-XPS or combustive elemental analysis: presence of heteroatoms; coal may contain small amounts of metal.

-Raman: D/G mode ratio

-TGA: gas content measurement

Here are the ranks for coal classifications:

https://en.wikipedia.org/wiki/Coal_analysis#Coal_classification_by_rank

Please be aware that the differentiation is made by national standardization organizations, so I don't know if Mexico uses the same ranks as the US.