How does a book prove an online wager?

[3put]

If you wager by phone there always is a tape proving the bet.

When I make a wager I take a screenshot of the bet-ticket.
That is my proof.

But suppose you find a losing online wager in your account, and you are certain you did not place that wager.
How does the book prove the wager has been made by you?

[ChuteBoxe]

First step is matching the IP addresses from the majority of your bets to the one bet in question.

[pokernut9999]

They do not have to, they already got your money.