Type plot( and then press the TAB key:
sage: plot(
Click on Help at the top of this page
There is also Live Documentation in which you can evaluate the examples.
Introspection:
sage: m = matrix([[1/2,1],[2,1]]); m [1/2 1] [ 2 1]
sage: m. sage: g = Graph(); g sage: g.
Looking at the sources:
.. skip::
sage: m.det