Many penises grow about 2.5", give or take a bit. The vast majority fall in the range of 1.75" to 3.25" growth. The belief that, in general, upon erection, "growers" catch up with "showers" is only partially true. A study done at Thundersplace's database, and another site with a smaller sampling to compare results with, both found that the mean erect overall average of 3" flaccids is 5.75" erect, and that the mean erect overall average of 5" flaccids is 7.25" erect.
What begins as a 2" difference when flaccid becomes a 1.50" difference when erect. The smaller flaccid closes the gap by one half of an inch. That is not a "catches up" however.
Of course that is just a general overview. There are smaller flaccids that catch up or even overtake bigger flaccids. That is not the norm though. This stuff about smaller flaccids are not smaller when erect is just one of very many pc posts I see around here.
Based on that, yes, one can decide quickly from seeing a flaccid size about what size it will 'likely' be when erect, give or take a bit. It can be a missed guess, but it much more often than not will be close to correct.