Purchase Request rules can be checked via the Financial Operations Administrator as below. The rule is applied based on who approves the request rather than the approver to whom it is assigned, unless nominated via FinOps:
Go to System options, Purchasing, Purchase Requests Setup.
Select the required item and click edit.
On the Approval tab, go to the appropriate stage and click Edit.
Depending on the message, select either the Pre or Post stage rules tab select the rule and click Edit.
The rule is held in the Expression field and once established, the information it relates to can be checked.