Detect Currency Forgery

Approaching the Exchange and Deposit Desk, the Wizard put down the sack of coinage. "I need to convert this to platinum, please."*

The spectacled dwarf mindlessly rattled off a stream of conditions. "You may exchange no more than 500 coins in a single transaction. The fee for a transaction of exchange or deposit is one coin of each type being submitted, which will be consumed and is non-refundable. Therefore one copper, one silver, one electrum, one gold and one platinum is consumed from your coinage for each respective metal that is present. Coinage of impurity or with any detectable magic will be arcanely consumed without explanation, discussion or refund." He stared at the wizard over his spectacles, "Do you understand what I have explained and are willing to accept the costs and risk?"

"The wizard stood blank-faced before speaking slowly.  "You know, I just remembered.  I left a cauldron of potions boiling back at home.  I'll come back another time.  No rush."

This cantrip is cast over a flat area of the coinage of a maximum of 4 square feet. No more than 500 coins can be cast over at once. The caster waves a hand from right to left the space. The coins flip and slide to the right. Any suspect currency jumps up and vanishes. The first real coin of each metal being scanned jumps up and is consumed by the cantrip. If the spell reaches 500 real coins, the remainder of the coins is pushed off to the left, leaving up to two piles. On the right are real coins, at far left unreviewed currency.

The cantrip will see as suspect any coin with any magical effect, polymorphisms, illusions or divine traces detected. Coins, where the material on the outside is not consistent through to the center and of less than 95% purity, are suspect. No further information about the nature of the suspect coins is understood. They are simply consumed to continue fueling the spell.

The spell's capacity increases by 500 coins when you reach 5th level (1,000), 11th level (1,500), and 17th level (2,000).

This cantrip was developed by a Dwarven Wizard in the Banking and Securing Industry.  
It made is way into the "Spells for the At-Home Caster" from someone who worked the Exchange Desk for 14 years.