How can you know a living organism is alive?
It would have been helpful to know if you were referring to plants or animals when asking this question, as indications differ. There are some generalities that can be used as indicators of life, however.
Living organisms have some sort of active respiratory function, allowing for intake of needed oxygen (in the case of animals) or carbon dioxide (for plants) and ridding the organism of wastes. All living things need "nutrition" of some sort, whether it be food eaten by an animal, absorption of nutrients from an outside source, or photosynthesis to create "food" within a plant for its own use.
Living organisms react to stimuli. This may be a very small movement or change in the organism itself or it may be a very large reaction to some outside condition.
Living organisms reproduce themselves. The methods by which this process is accomplished vary greatly, depending on the type of plant or animal you are considering, but the potential for it happening marks all living things.