Merchant fraud can often be detected by some kind of untypical behavior of the merchant. Deviations from historical averages of processed transaction volumes, abrupt change in processing patterns, untypical processing hours and entry modes, large numbers of even-amount transactions, long-term inactivity of the account – these are the criteria, signifying potential fraud.
If you have multiple merchants in your portfolio, and are financially liable for your merchants, keep an eye on all the aspects of their activity.
Find more information on this issue at the respective article at Paylosophy Blog.