"It's a fact. Look it up" was often said by several people, including Edna Strickland, when making a claim they believed to be true.
On May 14, 1986, Marty McFly denied Edna's accusation of him being a hooligan. Edna brought up his habit of arriving to school on skateboard in a very late fashion, and then stated "All skateboarders are hooligans. It's a fact. Look it up." She said it again, after apologizing, claiming that 9 out of 10 of the people in the city are hooligans.
In 1986B, Jennifer Parker preferred Leech to Marty as she deemed him a better guitar player. Marty asked if that really made him a better boyfriend. Jennifer responded by stating that "The better a guitar player, the better the kisser. It's a fact. Look it up."
Citizen Brown, after hearing several claims of Marty being a time traveler, eventually responded that he dedicated his life to form a more orderly society with the practical use of technology, and then finished by saying "It's a fact. Look it up."
The 1986B library Citizen Reading Room, had a poster with Edna on the building. Above the picture was her quote "Look it up".
On July 17, 1876, after Edna thought she heard a mouse (which was actually Marty stepping on a loose board), Beauregard Tannen asked if she was scared. She stated that she wasn't scared, but said "mice carry diseases. It's a fact. Look it up."