What criteria do linguists use to tell if something is a copula?
Let's say there's this group of undocumented languages and a linguist is studying nonverbal predication in those languages.
Mary bla-yi gul Mary COP-PRES teacher 'Mary is a teacher'
Mary sho gul Mary 3.SG teacher 'Mary (she) is a teacher.'
Mary sh=gul Mary 3.SG=teacher 'Mary (she) is a teacher.'
Mary nun gul Mary DIST teacher 'Mary (that) is a teacher.'
Mary ha gul Mary FOC teacher 'Mary is a teacher.'
We have a variety of items which can pass as copulas because they all seem to be linking the subject with the predicate. Language A has an actual verb (or auxiliary), Language B a pronoun, Language C a proclitic, Language D a distal demonstrative and Language E a focus particle.
Stassen (2013) states that there exist copulas that come in the form of verbal copulas, pronominal copulas, and particle copulas.
Is there a checklist that linguists use to tell if something is a copula? Do linguists identify copulas based on their linking function regardless of their form?