MOST mornings, commuters simply flash their monthly passes at Metro-North conductors with nary a word. Since May, however, conductors have begun demanding a more careful check of all riders as part of a fight against counterfeiting.

Although conductors have sporadically performed inspections for years, they are now examining all passes once a month. All legitimate tickets are then hole-punched to show their validity; fake passes are confiscated and those carrying them can be arrested. The railroad detailed its inspection plans in an April newsletter left on train seats on each of the three lines, said Marjorie Anders, a Metro-North spokeswoman.

The increased scrutiny coincides with the 25 percent fare increase put into effect May 1. The monthly fare from Tarrytown to Grand Central Terminal, for example, rose to $203 from $163. From a northern town like Peekskill, the monthly fare now costs $239. At the same time, improved scanning, copying and printing capabilities, all at lower costs, have lead to easier and cheaper counterfeiting.

Each of Metro-North's approximately 650 conductors has a unique hole punch, in shapes as basic as a heart or as esoteric as a parallelogram. (Some punches are considered so special, Ms. Anders said, ''that they get handed down upon retirement, especially in dynasties of conductors.'') The inspection program, which is essentially a low-tech method of combating high-tech fraud, appears to be supplementing the efforts of ''eagle-eye conductors,'' as Ms. Anders called them, who were already spotting the counterfeit passes. In the first half of 2002, only six people were arrested for fraud; during the same time period this year, 25 were charged, and of those, 8 have been arrested since May 1. Penalties can range from a fine to a maximum of one year in prison, Ms. Anders said. The penalties depend on where the counterfeiter is caught. Someone arrested in Connecticut with a forged ticket, no matter where he or she boarded the train, faces a fine of up to $1,000 and/or a year in jail. In New York, the penalties are a fine of up to $5,000 and/or up to five years in jail.