Mary has the moon in her hair
Susan has the star in her hair
Smell. Density test. Reactivity test. PH test.
Nothing. They are the same. There is no way to tell them apart.
The answer depends on what you want tell it apart from!
CAPTCHA is a tool that protects websites from bot attacks by running tests that tell humans apart from computer programs. The term is an acronym for 'Completely Automated Public Turing test to tell Computers and Humans Apart'.
it depends what you are telling apart
What do you mean exactly? Certainly you can perform a DNA test to see who the parents of a child are at any stage.
Meerkats tell each other apart with smell and dominant
Mary did not tell any one to write about Jesus.
Depends what "them" is
You can tell if a mineral is a mineral by its characteristics: *color *luster *streak *hardness *cleavage/fracture *crystalline structure
As humans we have a hard time telling penguins apart. However, They can tell each other apart by distinct vocalizations.
For the average human viewer there appears to be little way to tell a Pill Bug apart but they do have markings on there backs which can allow them to be told apart.