  Standing slow mode waves in hot flaring loops are exclusively observed in
spectrometers and are used to diagnose the magnetic field strength and
temperature of the loop structure. Due to the lack of spatial information, the
longitudinal mode cannot be effectively identified. In this study, we simulate
standing slow mode waves in flaring loops and compare the synthesized line
emission properties with SUMER spectrographic and SDO/AIA imaging observations.
We find that the emission intensity and line width oscillations are a quarter
period out of phase with Doppler shift velocity both in time and spatial
domain, which can be used to identify a standing slow mode wave from
spectroscopic observations. However, the longitudinal overtones could be only
measured with the assistance of imagers. We find emission intensity asymmetry
in the positive and negative modulations, this is because the contribution
function pertaining to the atomic emission process responds differently to
positive and negative temperature variations. One may detect \textbf{half}
periodicity close to the loop apex, where emission intensity modulation is
relatively small. The line-of-sight projection affects the observation of
Doppler shift significantly. A more accurate estimate of the amplitude of
velocity perturbation is obtained by de-projecting the Doppler shift by a
factor of $1-2\theta/\pi$ rather than the traditionally used $\cos\theta$.
\textbf{If a loop is heated to the hotter wing, the intensity modulation could
be overwhelmed by background emission, while the Doppler shift velocity could
still be detected to a certain extent.
