A study of the University of Barcelona, co-authored by Lena Hagenfeld, investigates how well humans discriminate visually presented gravities. The rate of change of the elevation angle and the visual angle were identified as major cues. 
Joerges, Bjoern, Lena Hagenfeld, and Joan Lopez-Moliner. "The use of visual cues in gravity judgements on parabolic motion." Vision research 149 (2018): 47-58. https://doi.org/10.1016/j.visres.2018.06.002