A memory can survive losing the person who could have confirmed it.

Some memories become uncheckable. The one other person who was there is gone, and now whatever you hold is the entire record, with no possibility of correction or confirmation. That is a particular kind of loneliness, and it does not make the memory false, only unverifiable and yours alone to carry.

Where it matters, write it down as fully as you can, including your uncertainty. And where you can still ask someone about a shared past, ask sooner rather than later. Most of the confirmation people wish they had was available for years and never requested.