Infall activity in high-mass star-forming regions is a topic of significant interest in astrophysics because it provides insights into the processes of star and planet formation. Studying infall, such as accretion streams or streamers, involves a combination of observational techniques and theoretical models.

Spectral Line Profiles:
Observations of molecular line profiles, especially from dense gas tracers, can reveal blue-skewed asymmetries, which are indicative of infall motions. The "blue asymmetry" in a spectral line profile results when the infalling gas, moving toward the observer, causes a doppler shift towards the blue end of the spectrum.
Commonly observed molecules for this purpose include HCO+ and CS.

Dust Continuum Emission:
High-resolution observations at submillimeter and millimeter wavelengths can trace dense dust and gas structures. These observations can reveal the presence of accretion streams or streamers feeding the protostar.

Infrared and Near-Infrared Observations:
Infalling material can heat up as it approaches the protostar, emitting infrared radiation. Observations in the infrared can trace these warm regions and, in some cases, reveal accretion shock fronts.

Maser Emission:
Certain maser lines, like those from water or methanol, can trace shocks and dynamic regions in star-forming areas. Their high brightness allows for very high spatial and spectral resolution observations, providing insights into small-scale dynamics, including possible infall activity.

Chemical Signatures:
The infall of material can induce specific chemical reactions due to changes in temperature, density, and UV radiation. Observing the presence or absence of certain molecules, or specific line ratios, can hint at infall processes.

Radio interferometers, like the Atacama Large Millimeter/submillimeter Array (ALMA) or the Very Large Array (VLA), allow for high-resolution observations that can trace infall structures down to small spatial scales, often revealing intricate details like spiral arms or streamers.

Numerical Simulations:
Computer models can simulate the dynamics of infalling material in star-forming regions. These simulations can be compared to observational data to test theories and hypotheses about the infall process.

Polarimetric Observations:
Polarization observations can trace the magnetic fields in the star-forming regions. Since magnetic fields can influence infall and accretion processes, these observations provide another avenue to study infall dynamics.

Variability Studies:
Monitoring the brightness of young stellar objects over time can reveal variability associated with episodic accretion events, providing indirect evidence for infall processes.