  Identifying active galactic nuclei (AGN) and isolating their contribution to
a galaxy's energy budget is crucial for studying the co-evolution of AGN and
their host galaxies. Brightness temperature ($T_b$) measurements from
high-resolution radio observations at GHz frequencies are widely used to
identify AGN. Here we investigate using new sub-arcsecond imaging at 144 MHz
with the International LOFAR Telescope to identify AGN using $T_b$ in the
Lockman Hole field. We use ancillary data to validate the 940 AGN
identifications, finding 83 percent of sources have AGN classifications from
SED fitting and/or photometric identifications, yielding 160 new AGN
identifications. Considering the multi-wavelength classifications, brightness
temperature criteria select over half of radio-excess sources, 32 percent of
sources classified as radio-quiet AGN, and 20 percent of sources classified as
star-forming galaxies. Infrared colour-colour plots and comparison with what we
would expect to detect based on peak brightness in 6 arcsec LOFAR maps, imply
that the star-forming galaxies and sources at low flux densities have a mixture
of star-formation and AGN activity. We separate the radio emission from
star-formation and AGN in unresolved, $T_b$-identified AGN with no significant
radio excess and find the AGN comprises $0.49\pm 0.16$ of the radio luminosity.
Overall the non-radio excess AGN show evidence for having a variety of
different radio emission mechanisms, which can provide different pathways for
AGN and galaxy co-evolution. This validation of AGN identification using
brightness temperature at low frequencies opens the possibility for securely
selecting AGN samples where ancillary data is inadequate.
