Honolulu police are warning the public about credit card skimmers after they were discovered at self-checkout registers at supermarkets across the island.

CrimeStoppers and the Honolulu Police Department in a Saturday news release said that the devices are attached to actual card readers and scan credit and debit card information when a card is used.

They can be placed anywhere credit or debit cards can be swiped.

The skimmers can be hard to spot, but they can be identified through a visual and physical inspection, CrimeStoppers said.

“Before using a point of sale terminal, ATM or gas pump, check for alignment issues between the card reader and the panel underneath it. Skimmers are often placed on top of the actual card reader making it stick out at an odd angle or cover arrows in a panel,” the CrimeStoppers news release said. “Compare the card reader to others at a neighboring terminal, ATM or gas pump and look out for any differences.”

CrimeStoppers also said a skimmer’s keypad and card reader can often help identify them. If the reader can easily come out of place, it may have been illegally installed. Also, authentic card readers are more robustly manufactured.

Police said that employees should be notified of any possibly tampered terminals. Those who may have been affected by the skimmers should call their credit card companies and look for fraudulent activities in their accounts.

Anyone with information about the skimmers is asked to call CrimeStoppers at 808-955-8300. The public may now send anonymous web tips to honolulucrimestoppers.org or via the P3 Tips app.