Former beauty contestant Cecelia D. Silva was found guilty yesterday in Montgomery County Circuit Court of theft and forgery, for using someone else's American Express card in a $6,000 shopping spree.

After a trial before County Circuit Court Judge Stanley B. Frosh and two days of deliberations, the jury found the Gaithersburg woman guilty of four counts of forgery and one of theft.

Silva was arrested last March, about one month after she had used the card to make four purchases at the Woodward and Lothrop department store at the Lake Forest Mall in Gaithersburg, according to court testimony. Silva had once worked in the store and was spotted by a security guard who knew her and noted that she was using a credit card issued in the name of Marian Ensor, prosecutors said.

According to court testimony, Robert Ensor, who is Silva's manager and is no relation to Marian Ensor, gave Silva the card, allegedly telling her it belonged to his former wife. Robert Ensor had intercepted the card when it was delivered mistakenly to his Gaithersburg apartment building, where Marian Ensor used to live. By then Marian Ensor had moved to a new address.

Silva testified in court that she believed the card belonged to Robert Ensor's former wife.

Silva has been a contestant in several beauty contests and in 1980 was named Miss Maryland-World.

Prosecutors said Robert Ensor also used the card for purchases for about six months after October 1981 when he intercepted it. Ensor has not been charged in the case.