In your deposit history at Coinbase, you can check all your previous transactions there. You can also check your email for this.
And for how you able to do it, just go to the Deposit page, choose the Bank options then from there, just follow the steps.
I am beginning to imagine that I did this with my debit or bank card.
Do you have an online account with your bank? I think there should be.
You can also check your transactions there. In most cases, it was a downloadable PDF and there's a password for it.