We have grown by molecular-beam epitaxy (MBE) GaSb1-xBix layers at different temperatures and fluxes to observe their influence on the Bi incorporation rate. All growth runs were monitored by in situ reflection high-energy electron diffraction (RHEED). Strong intensity oscillations were observed during the MBE growth of all GaSbBi and GaSb layers at very low temperatures, down to 170 degrees C (thermocouple temperature). We demonstrate that a detailed analysis of these RHEED oscillations gives an excellent insight into the variation of the Bi incorporation rate over the whole 2-13% composition range. While identifying the conditions to grow high quality GaSbBi alloys is a challenge generally addressed via a cumbersome trial and error approach, we demonstrate that RHEED oscillations is a powerful method to set the optimized growth conditions for the epitaxy of III-V-Bi alloys.