In Sean Carroll's GR book, the Lorentzian metric is defined as a metric $g_{\mu\nu}$ that when put in its canonical form,
$$g_{\mu\nu}=\text{diag}(-1,..,-1,+1,...,+1,0,...,0)$$ has no zeros and only a single minus.
It was also said that the metric determinant $g$ is always negative for a Lorentzian metric. I can see that this is true if the metric $g_{\mu\nu}$ is put into the canonical form as shown above. However, under an arbitrary coordinate transformation $x\rightarrow x'$, how can we be sure that the determinant $g'$ is still negative?