How do we see that Abigail is the acknowledged leader of this group in The Crucible?

You can see this at various points in the play. 
Basically, she tends to act like she is the boss.  She tells the girls what to do and
they pretty much obey her.

Just to give two
examples:

In Act I, Scene 1 the girls are kind of freaking
out, trying to figure out what to do.  Betty starts to accuse Abigail of drinking a
charm to kill Elizabeth Proctor.  Abigail "smashes her across the face" and tell her to
shut up.  Then she tells the others what they are going to
say.

Later, in Act III, Scene 3, when Mary Warren comes
with Proctor to the court to say that the girls are faking it, it is Abigail who starts
to "see" the big bird in the rafters and the girls all copy her.