  The most massive stars dominate the chemical enrichment, mechanical and
radiative feedback, and energy budget of their host environments. Yet how
massive stars initially form and how they evolve throughout their lives is
ambiguous. The mass loss of the most massive stars remains a key unknown in
stellar physics, with consequences for stellar feedback and populations. In
this work, we compare grids of very massive star (VMS) models with masses
ranging from 80-1000Msun, for a range of input physics. We include enhanced
winds close to the Eddington limit as a comparison to standard O-star winds,
with consequences for present-day observations of ~50-100Msun stars. We probe
the relevant surface H abundances (Xs) to determine the key traits of VMS
evolution compared to O stars. We find fundamental differences in the behaviour
of our models with the enhanced-wind prescription, with a convergence on the
stellar mass at 1.6 Myr, regardless of the initial mass. It turns out that Xs
is an important tool in deciphering the initial mass due to the chemically
homogeneous nature of VMS above a mass threshold. We use Xs to break the
degeneracy of the initial masses of both components of a detached binary, and a
sample of WNh stars in the Tarantula nebula. We find that for some objects, the
initial masses are unrestricted and, as such, even initial masses of the order
1000Msun are not excluded. Coupled with the mass turnover at 1.6 Myr, Xs can be
used as a 'clock' to determine the upper stellar mass.
