Even to discerning eyes, skimming devices aren't easy to spot. Especially when you're rushing to make a train. The MTA and the Nassau County DA's office say they have arrested four people on charges of stealing credit card information by using so-called skimming devices at LIRR stations. Thousands of credit and debit card numbers were found during the arrests. Authorities allege a married couple and two accomplices rigged nine ticket vending machines in Nassau County with scanners capable of reading card information. Investigators also found small pinhole cameras hidden above the keypad where PINs are entered. So far one bank has reported 50 accounts have been compromised. Since the discovery the MTA has twice surveyed all its vending machines. The second sweep found three machines in Westchester County were also rigged. Experts advise using a free hand to obscure the keypad when you enter your PIN.