s/he has been identified as a philosopher in any reputable, reliable encyclopedic/scholarly publication (e.g. MacMillan, Stanford, Routledge, Oxford, Metzler.)
s/he has written multiple articles published in reputable, reliable journals of philosophy and/or written books that were reviewed in such journals.
Reference works such as the following discuss the lives and summarize the works of notable philosophers:
(Cambridge)The Cambridge Dictionary of Philosophy, (Second Edition). Cambridge University Press; 1999. ISBN0-521-63722-8
(Macmillan2) 2nd edition (Donald M. Borchert, chief editor), 2006, ISBN0-02-865780-2
(Metzler)Metzler Philosophen Lexikon: von den Vorsokratikern bis zu den Neuen Philosophen, 3rd ed., Bernd Lutz (Stuttgart: Metzler, 2003). ISBN3-476-01953-5
(Oxford 1995)The Oxford Companion to Philosophy. Oxford University Press, 1995, ISBN0-19-866132-0.