  The expansion history of the Universe can be constrained in a
cosmology-independent way by measuring the differential age evolution of cosmic
chronometers. This yields a measurement of the Hubble parameter $H(z)$ as a
function of redshift. The most reliable cosmic chronometers known so far are
extremely massive and passively evolving galaxies. Age-dating these galaxies
is, however, a difficult task, and even a small contribution of an underlying
young stellar population could, in principle, affect the age estimate and its
cosmological interpretation. We present several spectral indicators to detect,
quantify and constrain such contamination in old galaxies, and study how their
combination can be used to maximize the purity of cosmic chronometers
selection. In particular, we analyze the CaII H/K ratio, the presence (or
absence) of H$\alpha$ and [OII] emission lines, higher order Balmer absorption
lines, and UV flux; each indicator is especially sensitive to a particular age
range, allowing us to detect young components ranging between 10 Myr and 1 Gyr.
The combination of these indicators minimizes the contamination to a level
below 1% in the case of ideal data. More importantly, it offers a way to
control the systematic error on $H(z)$ as a function of the contamination by
young stellar populations. We show that for our previous measurements of the
Hubble parameter, the possible bias induced by the presence of a younger
component is well below the current errors. We envision that these indicators
will be instrumental in paving the road for a robust and reliable dating of the
old population and its cosmological interpretation.
