Nancy Reagan has been checking with a friend in Rome about proper attire for the various events of her brief stay there in May. The visit with the pope calls for a hat and long black dress. When Jacqueline Onassis visited the pope years ago she changed her clothes in the airport ladies room with designer Valentino on hand to make sure she looked just right.

Valentino will be on hand for Mrs. Reagan, too, but only as a guest at the American embassy for dinner in the first lady's honor. Ruth Raab, wife of the American ambassador, has invited two other Rome-based designers -- Karla Fendi and Laura Biagotti -- to the seated dinner for 60.

So far there is no fashion show planned for Mrs. Reagan's visit. Lidia Gromyko, who was in Rome last month, got a tour of Biagotti's castle and a viewing of her designs. After seeing the range of white cashmeres that she specializes in, Biagotti said, Lidia Gromyko concluded, "They would look nice on some women in Siberia."

Biagotti gave Lidia Gromyko a Bulgari coin necklace as a souvenir of her visit. The designer received an embroidered Russian shawl in return.

Biagotti, whose white and pastel cashmeres looked best in her collection shown here last week, said her designs were being considered for Aeroflot uniforms.