It is assumed that the Sword absorbed basilisk venom when Harry killed the basilisk using the sword in the Chamber of Secrets, but is there any proof that this was the point when the sword got imbued with the venom? The sword is presumably hundreds of years old, and has probably been absorbing things that make it stronger. Isn't it possible that the sword might have absorbed basilisk venom at some other point? I understand that basilisks themselves are probably very rare, but the Sword is also at least a thousand years old. Also, when Harry stabbed the basilisk with the sword, it is described in the book (corroborated by the movie scene) that the sword was stabbed through the roof of the basilisk's mouth, while a fang entered Harry's arm and broke off. I always presumed that the basilisk had it's venom mostly in the fangs and not the blood. It doesn't seem likely that the Sword came in contact with enough amount of venom (following the description of the stabbing of the basilisk).
How can it be definitely proved (using canon descriptions) that the event in the Chamber of Secrets (stabbing of the basilisk) was the point at which the Sword absorbed the basilisk venom, and it did not already have some venom to begin with?