I've just saved a GeoTiff image via GDAL (C++ version). I've called BuildOverviews on it. How can I check if the pyramids are actually created in the image?
Fastest way without loading it into a GIS would probably be using gdalinfo from command line. That will list the overviews (pyramids) for the raster. Building overviews will also increase the file size and change the modification timestamp, so if you see that, you can be pretty sure it built the pyramids.