Quasi-Periodic Oscillations (QPOs) observed in the X-ray flux emitted by
accreting black holes, are associated to phenomena occurring near the horizon.
Future very large area X-ray instruments will be able to measure QPO
frequencies with very high precision, thus probing this strong-field region. By
using the relativistic precession model, we show the way in which QPO
frequencies could be used to test general relativity against those alternative
theories of gravity which predict deviations from the classical theory in the
strong-field regime. We consider one of the best motivated strong-curvature
corrections to general relativity, namely the Einstein-Dilaton-Gauss-Bonnet
theory, and show that a detection of QPOs with the expected sensitivity of the
proposed ESA M-class mission LOFT would set the most stringent constraints on
the parameter space of this theory.