"show that the account was funded"
By "account" in your original post, I take it to mean the IRA account.
In the absence of Form 8606, IRS will accept other suitable evidence to support your position.
Otherwise, the IRS can declare the entire basis to be zero.
they may or may not do this depending on their audit resourses when you file your return(s). and what mood they're in.
In other words, hold on to your evidence in case your "prior years basis in inherited IRAs" is disallowed.
Note: as you should know, inherited IRAs must be treated separately from your own IRAs.
@glerbnik84
