Here you go:
Code:
$ calibre-debug -g
[...]
logicalDpi: 48.0 x 48.0
physicalDpi: 87.0857142857 x 85.725
[...]
Using Grollicus' suggestion and setting the QT_SCALE_FACTOR environment variable:
Code:
$ QT_SCALE_FACTOR=1 calibre-debug -g
[...]
logicalDpi: 96.0 x 96.0
physicalDpi: 174.171428571 x 171.45
[...]