You can utilize the Shipping Date field to know if your invoices are already shipped based on the date entered. However, this doesn't mark the invoice as shipped.
If you want to specify invoices that are being shipped, you can consider adding a custom field named "Shipped." From that field, you enter Yes if it's already shipped. Here's how:
First off, let's add a custom field to your invoice.
Click the Gear icon, then select Account and Settings.
Select Sales from the left menu.
Go to the Sales form content section, then select Custom fields.
Enter Shipped in the Name box.
Put a checkmark on the Internal and Public checkbox.
Click Save, then Done.
Then, find and open your invoices that already shipped and enter Yes to the Shipped field. I've attached a screenshot below for your visual reference.
Once done, you can run and customise the Invoice List report to show invoices that are being shipped. For more guidance, feel free to check out this article: Customise reports in QuickBooks Online.