I'd show cancelled checks and stuff like that....could be wire transfers or receipts in the closing docs and the like.
The gift tax thing is a gift tax return because no gift tax would be owed if your father was under the $11,400,000 exclusion amount and if over you should get a lawyer anyway.....but I don't know how there would be a gift after reading Pub 551 because a completed gift would be removed from the estate.....looks like Pub 551 says it's not removed from the estate but sort of inherited by the joint tenant with a step up in basis according to the percentage contributed by the deceased.
