Well, now we know the truth about Ronald Reagan.

Steve Martini, barber to four presidents, dropped in yesterday to say that he has personally resolved the most asked question of the 1980 campaign. When Reagan visited Boca Raton, Fla., where Martini runs the hotel hair-dressing salon, the veteran hair razer made a point of standing close behind the candidate.

"I can tell you: He doesn't dye it," Martini said firmly. "The roots are not white."

It is conceivable, however, that he rubs in something like Grecian Formula. As it happens, Martini is promoting Grecian Formula these days.He uses it himself, and he looks about 50, which is pretty good for 68.

Reagan, he observed, sports the same conservative cut he used for decades as a cowboy actor and even earlier -- "if you can remember that far back" -- and he probably wouldn't look as good with longer hair. In fact he might take a bit off the top (to get rid of that curious lump on his skull). His clothes are perfect, said Martini -- like Kennedy's.

As for President Carter, "He must have a pretty fair stylist, but he needs to get his hair a bit darker. He would come across as a stronger person. Now he has that tintype look, all white and washed out. His clothes are impeccable."

John Anderson could use a restyling, "and a good rinse, to get the yellow out." The thatch of white hair could be shaped to his head, too, Martini reflected.

There have been a lot of changes in 50 years of babering. They had 53 chairs in the barbershop of the old Pennsylvania Hotel in New York when Martini started out. Then the safety razor came along and men began to shave themselves. Today there are two chairs in that shop, and one barber.

"It's all hair dressing today, there are hardly any barbershops as such," he said. "We do permanents and tinting and straightening and streaking in our place. Men are getting more conscious of grooming."

It was the Beatles who started long hair among the young, he said, making it a symbol of resistance against whatever the young were resisting at the time. But in uptight establishment Washington, it was Lyndon Johnson who led men to new lengths, if not heights. When he left office he grew his hair clear to his shoulders, and Martini once phoned him to tell him to, for goodness sake, go get a haircut.

Today in urban America, he observed, the short Ivy League cut belongs to the gays, often trendsetters.

Ike would flip, he added.

Martini was thick with several presidents. His son Steve got to swim in the White House pool with LBJ. Nixon flew Martini, his personal barber, to Florida once to visit his ailing mother. He cut Kennedy's hair the day before Dallas. His presidential career began in 1959, when the Secret Service decreed him trustworthy to cut Eisenhower's hair because he had cut Gen. George Marshall hair at the Pentagon, and nothing terrible had happened to Marshall.

But in 1971 Martini hit an air pocket pleaded guilty to charge of embezzling $15,000 from an estate for which he was trustee and income tax violation involving his employes' withholdings, and left the Washington scene. He has built himself a new life in Florida, surrounded by his wife and three children, and he says he feels younger all the time, with that good old Grecian Formula.