The Supreme Court, acting in a case from Hyattsville, agreed yesterday to decide whether police have the power to determine what is obscene and to arrest bookstore owners and close their businesses.

The justices accepted an appeal by Maryland Attorney General Stephen H. Sachs, who asked the high court to reverse a state appeals court ruling last March that threw out the 1981 conviction and $500 fine of Baxter Macon, an employe at a bookstore called the Silver News Inc.

The appeals court said that Prince George's County detectives, who were conducting a countywide crackdown on adult bookstores, violated Macon's rights to free speech. The appeals court ruled that the detectives should have first asked a judge to determine whether the materials were obscene and then asked for an arrest warrant.

Sachs, in his appeal, said Maryland law allows police to arrest people who commit misdemeanors in their presence or when they have reasonable cause to believe a misdemeanor has been committed in their presence.

The appeals court, he argued, created an exception to that law by "judicial fiat," by saying that a police officer can never arrest smut peddlers without a warrant.

Sachs argued that even if Macon's arrest was illegal, the magazines could still be used as evidence because they were purchased, not seized, by the detectives before the arrest. The case, Maryland v. Macon, is only the second obscenity case the court has agreed to hear this year.

In another Maryland case, the justices agreed to hear whether Baltimore's firefighters may be forced to retire at age 55 without the city having to show that age prevents them from doing their jobs.

Under the Age Discrimination in Employment Act, no one under 70 years old can be forced to retire unless employers show that age is a "bona fide occupational qualification."

The 4th U.S. Circuit Court of Appeals in Richmond said Congress has set 55 as the retirement age for federal firefighters, so Baltimore was merely following a congressional determination that 55 was the appropriate age.

Six firefighters, joined by the Reagan administration, urged the high court to overturn that ruling.

The case is Johnson v. Mayor and City Council of Baltimore and Equal Employment Opportunity Commission v. Mayor and City Council of Baltimore.

In another discrimination case, the court declined to hear Northwest Airlines' appeal of a $60 million award to about 3,300 female flight attendants who sued the company for sex discrimination.

The female flight attendants argued that the airline paid them less than male pursers for identical work and discriminated against them on fringe benefits and in other ways. The federal appeals court here has ruled three times in the heavily litigated case -- first in 1976, then in 1980 and most recently last July.

The high court's action yesterday clears the way for the women to receive between $15,000 and $60,000 each in back pay and interest and appears to end the 15-year-old case. But the battle between the airline and the women's lawyers over attorneys' fees, which could amount to at least $2 million, is not resolved. The case is Northwest Airlines v. Laffey.

The court agreed to decide whether a man who mailed "bootleg" Elvis Presley records from Maryland to buyers around the country can be prosecuted for interstate transportation of stolen property and mail fraud.

The records were manufactured illegally from copyright-covered soundtracks of Presley's movies, television and concert appearances and studio outtakes. The case is Dowling v. U.S.