  Elliptically polarized laser pulses (EPLPs) are widely applied in many fields
of ultrafast sciences, but the ellipticity~($\varepsilon$) has never been in
situ measured in the interaction zone of the laser focus. In this work, we
propose and realize a robust scheme to retrieve the $\varepsilon$ by temporally
overlapping two identical counter-rotating EPLPs. The combined linearly
electric field is coherently controlled to ionize Xe atoms by varying the phase
delay between the two EPLPs. The electron spectra of the above-threshold
ionization and the ion yield are sensitively modulated by the phase delay. We
demonstrate that these modulations can be used to accurately determine
$\varepsilon$ of the EPLP. We show that the present method is highly reliable
and is applicable in a wide range of laser parameters. The accurate retrieval
of $\varepsilon$ offers a better characterization of a laser pulse, promising a
more delicate and quantitative control of the sub-cycle dynamics in many strong
field processes.
