A tasting knife is a light (approx. 50 g) long (approx. 25 cm) and narrow (approx. 1 cm) pocket knife that is used to taste sausages and other fine meats at fairs, etc. (1). The blade is often partially serrated to easily cut the sausage. [MOT]
(1) According to SARGENT: 18, the knife would be used to test melon or citrus fruits.