The following are the appropriate fields on the table which stores the data:

Approval Notify -1 means that it has been assigned and the "Approved By" label will go red.

F3 Purchase Order assigned for approving on this date.
F3 Purchase Order assigned by this person for approving.
F3 Purchase Order assigned to this person for approving.

Note that Approval Notify is 0 until F3 assign. Then it goes to -1 and stays -1Approver and logged in user when approved.
Purchaser and logged in user when purchased.

Note that you cannot Approve a Purchase Order with no lines

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