  Early optical afterglows have been observed from GRB 990123, GRB 021004, and
GRB 021211, which reveal rich emission features attributed to reverse shocks.
It is expected that Swift will discover many more early afterglows. Here we
investigate in a unified manner both the forward and the reverse external shock
emission components, and introduce a straightforward recipe for directly
constraining the initial Lorentz factor of the fireball using early optical
afterglow data. The scheme is largely independent of the shock microphysics. We
identify two types of combinations of the reverse and forward shock emission,
and explore their parameter regimes. We also discuss a possible diagnostic for
magnetized ejecta. There is evidence that the central engine of GRB 990123 is
strongly magnetized.
