Hi @APropst,
Yes, there is a way you can identify who marked a particular task as complete or even manually make any changes to a task in your HubSpot. However, you need to be a super admin to be able to do this.
In your HubSpot, hover over Sales → Tasks. Then, click on “Add view” and select “Completed”.
Please make sure all other filters (assignee, task type, due date, etc) are empty/unchecked to cover every user in your account and give you accurate data.
Let me know if this helps.