The truncation error can give an idea of the error in the energy, but the main strategy is to run DMRG at various bond dimensions and plot the energy as a function of the bond dimension or truncation error and extrapolate the energy with a polynomial fit. The error bars from this procedure are not well defined, but strategies I have seen are:
1. Determine the error bar from the difference between the energy obtained from the largest bond dimension (which gives an upper bound of the energy) and the extrapolated value).
2. Perform extrapolations using different sets of data points and obtain an error bar from the range of extrapolated values obtained.
Another strategy is to use the variance of the energy, but that is more expensive since it involves computing <H^2>, so it is not used often.