Hello there, abmfoodsinc.
In QuickBooks Online, there's no specific list that shows which employee already received W-2 in the mail.
If you have sent it via mail, I'd suggest it with your courtier to confirm it. On the other hand, if your employee has access to Workforce, they can access the W-2 after you e-filed the form.
Here's some resource to help you get ready with tax season:
Comment below if you have other concerns with QuickBooks. I'd be happy to work with you again. Stay safe and be well!