The densities of states, eigenfunctions, and lifetimes of the Stark resonances arising in the conduction band of (Al,Ga)As/GaAs/(Al,Ga)As quantum wells under strong electric fields are determined. A quantitative comparison between the Al-concentration-dependent and -independent effective-mass approximations is also provided. A numerical real-time wave-packet propagation method, successfully implemented earlier for bound states, is adapted for this task by supplementing the Hamiltonian with an exterior optical potential, which damps the wave function in the asymptotic region. Such a technique permits both the reflection-free simulation of the continuum with a finite computational grid and the filtering of the nonresonant background out of the eigenspectrum. This approach yields the entire resonance spectrum of a given structure in a single calculation and ${L}^{2}$ approximations to the continuum eigenfunctions. The lifetime of a resonance is obtained from the time-decay constant of the norm of the calculated ${L}^{2}$ resonance eigenfunction, a method that is applicable to resonances of any line shape.