Andrea Rossi has said no more public tests.
http://freeenergytruth.blogspot.com/2011/11/rossi-no-more-ecat-public-tests.html
It seems like it could be tested by taking a sample from the device before it ran and another sample after it ran and compared their element/isotope ratios.Rossi did exactly that. His "before" sample contained Nickel, and his "after" sample contained Nickel, Copper and Iron. But they were all identical to naturally-occuring elements, with no signs of nuclear processes having occurred.
From Wikipedia:
As Ny Teknik reports, Peter Ekström, lecturer at the Department of Nuclear Physics at Lund University in Sweden, concluded in May 2011, "I am convinced that the whole story is one big scam, and that it will be revealed in less than one year."[22][23] He cites the unlikelihood of a chemical reaction being strong enough to overcome the Coulomb barrier, the lack of gamma rays, the lack of explanation for the origin of the extra energy, the lack of the expected radioactivity after fusing a proton with 58Ni, the unexplained occurrence of 11% iron in the spent fuel, the 10% copper in the spent fuel strangely having the same isotopic ratios as natural copper, and the lack of any unstable copper isotope in the spent fuel as if the reactor only produced stable isotopes.[22] Kjell Aleklett, physics professor at Uppsala University, said the percentage of copper was too high for any known reaction of nickel, and the copper had the same isotopic ratio as natural copper. He also stated, "Known chemical reactions cannot explain the amount of energy measured. A nuclear reaction can explain the amount of energy, but the knowledge we have today says that this reaction cannot take place."[24][23]A recent paper by Miley (PowerPoint) showed that their LENR experiments are producing 39 non-natural isotopes. Rossi's device is producing 0 such isotopes.