Distance between first barline and paper (in preview)

[Urs Liska]

Is there any possibility to determine the exact x-position of the 
leftmost barline in a -dpreview example (well, apart from opening it up 
in a graphics program and measure)?

I would like to try out to left-align (or justify) music examples in a 
text document on the barline, so the piano or staffgroup brackets etc. 
protrude into the left margin.

I would equally use a way to determine that value from the finished 
document or to set it explicitely.

TIA
Urs