  A Monte Carlo simulation based on O(alpha_s) QCD matrix elements matched to
parton showers shows that final-state hadrons in deep inelastic scattering
(DIS) can be used to tag events with a single (anti)quark recoiled against the
proton. The method is particularly suited to study the mean charge of leading
particles, which is sensitive to fragmentation and sea quark contribution to
the proton structure function. We also discuss methods to study the charm
production in DIS using the Breit frame.
