Unfortunately, Square doesn't really have an easy way to distinguish online sales versus in-store sales. I have come across a few workarounds though...

• The quick and dirty way is to look at your Payment Methods report. If you're not using Invoicing or any other external credit card collection methods, what you see in the Other category will be your online sales.
• If you look at your Employee Sales report, look at the Untracked Employee line. As with the Payment Methods report, as long as there are no other payments coming in that will be your online sales
• If you're using the Square Online Store you can download the CSV of transactions from the Online Store main screen in the dashboard
• You could set up a Custom Report showing all Untracked Employees sales

To be perfectly honest, none of those solutions are the best bet. If you're wanting to see definite data on your online sales, I recommend setting up items specific to the online store. I put those items in a category that starts with OL so I can pull specific sales reports. Yeah, it's a lot more time consuming to do this and you'll have duplicate items in your library, but you have a lot more of an idea how your revenue centers are doing.