There are various controls you can put in place, to easier detect suspicious invoices and fraudulent vendors. For instance, make it mandatory for the Purchasing department to check all new vendors, require pre-approved POs for all purchases and use multi-level approvals for invoices above a certain amount.
Another way to identify fraud in accounts payable is using automation, such as automated invoice capture which identifies duplicates and new vendors in few seconds, or intelligence (AI) to detect suspicious patterns in incoming invoices and workflows.