Short answer. As of the date of death. Article 195 states that all questions of relationship and dependency are determined as of the time of death, so the position is fixed on that day. What changed in your circumstances afterwards does not alter what you were when he died.
What the law says
All questions of relationship and dependency shall be determined as of the time of death.
Labor Code, Article 195 — Relationship And Dependency. Read the full provision →
The date freezes the picture
Article 195 is one sentence long and does one thing: All questions of relationship and dependency shall be determined as of the time of death. Both questions are answered by the same moment. Who stood in the necessary relationship to your father, and who was in fact dependent on him, are settled by how things were on the day he died. Everything before that date is history that explains the position; everything after it is a change in your life that the article deliberately declines to take into account, in either direction.
Later changes do not take an entitlement away
This cuts both ways, and for most families the protective side is the important one. A dependent who afterwards finds work, moves in with relatives, receives help from a sibling, or comes into money from some other source has not thereby become undependent as of the date of death. Claims are often assessed months later, when a household has necessarily rearranged itself to survive, and the temptation is to describe the family as it is now. The article asks about a different day.
And it does not create one either
The same rule closes the door the other way. Someone who was not dependent on your father when he died does not become so by falling on hard times afterwards, and a relationship formed after the date of death is not brought within the article by later events. This is not harshness for its own sake. A scheme that paid according to circumstances as they stood at the moment of assessment would invite claims to be timed, and would leave nobody able to say with certainty who was entitled.
So the proof is about that day
Gather what shows the household as it stood then, not now. The death certificate fixes the date. Birth and marriage certificates establish relationship. For dependency, the useful material is ordinary and often overlooked: remittance records and bank transfers, school receipts and enrolment records in his name, the address on identification cards and bills, medical papers showing who cared for whom, and statements from people who knew the household. Where circumstances changed shortly afterwards, say so plainly and date the change, rather than describing the present as though it were the past.