During each lunar cycle, the full moon is technically full for one instant, but to the human observer, the moon appears full for 2-3 days and nights in a row, making it hard to tell on which date and time the full moon is, just by looking at how round the illuminated area of the moon appears.
For example, in the image below, the moon appears to be at its fullest on the 23rd or 24th, but it's hard to say exactly when.
But if you were in Miami, Florida, USA at the right hour on the dates below, with a clear view of the entire sky, you could tell with only simple, natural sky observations during that hour that the moon was full “around now, plus or minus a couple of hours”. What type of observation could you make on each date? It is different on each date. No instruments, calculations, or reference data are required during the time of the observations (you'll probably need to look up some info to solve this puzzle though).
March 1, 2018
January 21, 2019
May 7, 2020