I think the first and most obvious way to check would be according to
Unicode Version, i.e. check for some Emoji introduced in version 6, in
version 7, and so on. For very old sets, checking for emoji present in
the NTT Docomo set but not in the Softbank set,... might also make sense.
oh, I should add more words why I wrote "subset". There is a full
list of emoji defined by Unicode;
http://unicode.org/Public/emoji/3.0/emoji-data.txt
But I'm questionable whether the most emoji font developers are
trying to fill all of this list.
For example, to check the support level for
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