Are there any (visible) differences between R1 and R2 DVDs (or blu-rays, thereof)? I was wondering whether there is any visible difference between Region 1 and Region 2 movies. Region 1 is encoded in 23.976 fps standard, right? And Region 2 is 25 fps.

But as I assume the movies are shot in 23.976 only, the R2 releases are just some convert/recode to be compatible with the region.


  Which ultimately leads me to a question: If presented with a movie
  playing from R1 and R2 discs simultaneously, can a trained eye see any
  difference?


I know for a fact an eye can see the difference between 23.976 fps and 25 fps. A lot of low-budget movies/web series are being released lately since the Canon dSLRs can record video and there definitely is a visible difference between the standard 24 film fps and the "fluid, unnatural" 25 fps.
There is no visible difference, unless the DVD/BR publisher labels their discs as such.  Any region code (or even region-free data) can be burned or pressed on to any media.

Furthermore, region encoding (at least on DVDs, and I assume on BRs) has nothing (technically) to do with the video format.  Now, you're likely to find Region-1 DVDs in NTSC format, Region 1 is the U.S. and Canada, and the U.S. and Canada use NTSC, but there's no technical barrier to creating a R1 DVD in PAL format.

See the Wikipedia article on DVD Region codes, and specifically the section on NTSC vs. PAL.