The best sign would be some form of metabolism. Gases should be exchanged (respiration) from example oxygen should be converted into carbon dioxide. Some chemical compound should be converted and broken down for energy such as glucose to carbon dioxide and water. Further signs that something is living: reproduction, movement (obviously not with plants), reaction and interaction with the environment.
To be classified as a 'living thing' an organism and its cells must make use of all seven of the seven biological life processes: Movement, Reproduction, Sensitivity, Growth, Respiration, Excretement and Nutrition.
Every living thing have some metabolism going on due to the breakdown of food ingested being converted into energy needed for locomotion and other important body functions. this is a special characteristic for living things only.
Living things have basic functions, like some kind of respiration, the taking in of fuel (or nourishment), a form of reproduction, and the disposal of waste. Living things include both plants and animals.