The first attribute they look for is the aged clay. Given enough examples of authentically aged and recently aged clay, this task is fairly easy.
Aged clay is usually black, but is often a mix of black and red.
Most sculptures created in the 17th to 19th centuries were filled with red clay and display a greater proportion of black clay the older they are. In order to simulate the aged density, fakes are usually either filled with baked clay or cement. Ratanagosin and Raachagaan Period Buddhas will normally look very similar on the underside to the bottom of this huge mid-19th century Ratanagosin Buddha, which is about 2 feet across. Although most characteristic of Thai Ratanagosin sculpture, we have observed a double cast layer of bronze on sculptures dating from the 15th century to the 19th century. The double-layered bronze is virtual proof of authenticity, as a replication created using such old world methods would cost significantly more than the genuine item, but sometimes evaluation is a bit difficult if the edge has been smoothed over time, hiding evidence for the double layer.