There is no single definition of life that philosophers of science have agreed upon. In the absence of a definition most biology textbooks describe life: they list characteristics of living things, but these lists are all different. And, no matter which list you pick, natural history enthusiasts are always ready to point out a living or nonliving thing that the list puts in the wrong category.
Many lists of the characteristics of living things describe what a living thing would need to do to postpone entropy (Schrödinger, 1944). For example, life must be able to 1) use energy from some source to drive chemical reactions, 2) reproduce, and 3) undergo evolution (Jakosky, 1998).
Living things need parts to accomplish life processes like eating, reproducing, growing, and moving. These parts can be observed and identified as they change over time. It is important to observe while interfering as little as possible, so making observations in a living thing's home environment can be very helpful.
Life processes may follow a predictable history, or cycle, as living things invest different amounts of energy into different processes. Changes in conditions (e.g. seasons, weather, tides, the proximity of predators) force living things to change the amount of energy they invest in processes (e.g. eating, reproducing, growing, and moving) in order to maximize their chance to pass on their genes. Things that live in places with fast, dramatic, unpredictable changes tend to mature quickly, reproduce early, and spend less time caring for their young. Things that live in places with slow, minor, predictable changes tend to mature slowly, reproduce late, and spend more time caring for their young.
The parts of a living thing change as it moves through its life cycle. A living thing may look different during a period in which it invests more energy into eating than one in which it invests more energy into reproducing or moving.
Schrödinger, Erwin (1944). What is Life? Cambridge University Press. ISBN 0-521-42708-8.
Bruce Jakosky (1998). The Search for Life on Other Planets. Cambridge University Press. ISBN 0-521-59837-0