A turning-point in Beck's career came in 1991 when, as the world's authority on Jacopo, he was invited to comment on a recent restoration of one of the sculptor's works, an effigy of Ilaria del Carretto in Lucca Cathedral. Beck's enraged response was recorded by two reporters for newspapers based in Florence and Livorno, and in further interviews published in La Stampa (of Turin) and Il Giornale dell'Arte (based in Alessandria) he continued to voice his indignation at what he deemed a ruinous cleaning job. The sculpture's restorer filed lawsuits against him in the courts of all four cities citing criminal defamation, an offence punishable by three years' imprisonment in Italy.
Eventually, Beck won all four cases, and his success prompted him to establish ArtWatch International the subsequent year. The organisation's other founding member was Michael Daley, a British art historian with whom Beck co-wrote Art Restoration: The Culture, the Business and the Scandal in 1993. The book was an attack on the profession of art restoration, and was especially critical of the cleaning of Michelangelo's Sistine Chapel frescoes which was at that time nearing completion. Beck also publicly criticised the attribution of both the Metropolitan Museum's Stroganoff Madonna (which it bought in 2001) to Duccio and the Madonna of the Pinks (the National Gallery's major acquisition of 2004) to Raphael. He argued for the de-attribution of these works in his study From Duccio to Raphael: Connoisseurship in Crisis, published posthumously in 2007.