How do an Approval Manager come to know whether it is parallel or sequential multi approval when record is moved for approval?

if it is a parallel approval, all the approval managers will get the approval request. There will be an icon provided in New UI record list view called Pending approvals. Approval manager can click on the icon to view the other approval manager details and who has approved/Rejected the request. We can view the reason/comments provided by them.

If it is a sequential approval, the approval request will move to the next approval manager in sequential manner.

Also, the approval info can be viewed in Edit record screen.