If your distribution from the Roth 401(k) would not have been a qualified distribution if not rolled over, your basis in Roth IRA contributions is the same as your basis in Roth 401(k) contributions. If the distribution from the Roth 401(k) would have been a qualified distribution if not rolled over, the entire amount rolled over became your basis in Roth IRA contributions. Box 5 of the Form 1099-R that reported the rollover from the Roth 401(k) should show the amount of contribution basis your Roth IRA received.
