There are two places that this information can be found: on the individual offer or the Offer Details report.
Note: This information is only visible to Job Admin and Site Admin with the permission "Can see private notes, salary info, manage offers, and approve jobs/offers."
If you're looking for a list of approvers of a specific offer, a list of approvers can be found on the candidate's profile under the Offer Details tab. The Approval Details, under the Offer Documents section, will list all the approvers for that offer.
Offer Details report
If you're looking for a list of approvers for more than one offer, this information can be found under the Offer Approvers column in the Offer Details report.