While posting a foreign currency document, give a different rate.
The system will post a foreign currency exchange rate difference at payment time - comparing the rate of the invoice versus the rate maintained in SAP (OB08)
That'll show if the configuration (FBKP) is done correctly -- Auto Posting --> Foreign Exchange Differences --> check for KDB and KDF.
Also make sure to check valuation area, once you click KDB, the yellow box appears, where valuation area dependent accounts are maintained.