Transverse deflection structures are useful devices for characterizing the longitudinal properties of bunches in electron accelerators. With efforts to produce ever-shorter bunches for applications such as external injection into novel accelerator structures, e.g. plasma cells or dielectric structures, the applicability of deflection structures to measuring ultrashort bunches has been considered. In this paper, charge-density and bunch-length measurements of femtosecond and subfemtosecond bunches at the ARES linac at the SINBAD facility at DESY are studied with simulations and the limitations discussed in detail. The novel polarizable X-band transverse deflection structure (PolariX-TDS) will allow the streaking of bunches at all transverse angles, making a 3D charge-density reconstruction of bunches possible, in addition to the standard 1D charge-density reconstruction and bunch-length measurements. These various measurements of the charge-density distributions of bunches have been simulated, and it is shown that useful information about ultrashort bunches down to subfemtosecond lengths may be obtained using the setup planned for the ARES linac.