  We study two experimental ways to measure the heavy-quark content of the
proton: using the Callan-Gross ratio $R(x,Q^2)=F_L/F_T$ and/or azimuthal
$\cos(2\varphi)$ asymmetry in deep inelastic lepton-nucleon scattering. Our
approach is based on the perturbative stability of the QCD predictions for
these two quantities. We resume the mass logarithms of the type
$\alpha_{s}\ln\left( Q^{2}/m^{2}\right)$ and conclude that heavy-quark
densities in the nucleon can, in principle, be determined from data on the
Callan-Gross ratio and/or azimuthal asymmetry. In particular, the charm content
of the proton can be measured in future studies at the proposed Large
Hadron-Electron (LHeC) and Electron-Ion (EIC) Colliders.
