  A generalization to the quantum case of a recently introduced algorithm (Y.
Tomita and Y. Okabe, Phys. Rev. Lett. {\bf 86}, 572 (2001)) for the
determination of the critical temperature of classical spin models is proposed.
We describe a simple method to automatically locate critical points in
(Quantum) Monte Carlo simulations. The algorithm assumes the existence of a
finite correlation length in at least one of the two phases surrounding the
quantum critical point. We illustrate these ideas on the example of the
critical inter-chain coupling for which coupled antiferromagnetic S=1 spin
chains order at T=0. Finite-size scaling relations are used to determine the
exponents, $\nu=0.72(2)$ and $\eta=0.038(3)$ in agreement with previous
estimates.
