According to Napoleon Mabaquiao Jr., in the strict use of the word philosophy in the academe (strict use of philosophy), there are three fundamental hallmarks of philosophical works:

1. Topic: a philosophical work asks questions about foundations
2. Methodology: a philosophical work uses methods from philosophical traditions
3. Arguments: a philosophical work uses argumentation to advance ideas

However, I think these three fundamental hallmarks of philosophical works are restricting.

References

Mabaquiao, N. (2011). Isang Paglilinaw sa Kahulugan at Kairalan ng Pilosopiyang Filipino = A Clarification in the Existence and Meaning of Filipino Philosophy. Malay, 23(2), 39–56.