How to inspect a css file generated by Less?
There are extensions available for Firefox (sadly not for any other browsers that I can find).

There's the FireLess Plugin which is:


  a Firebug extension that makes Firebug display the Less filenames and line numbers of LessPHP-generated CSS styles rather than those of the generated CSS


Also, if you're able to switch to using SASS (which is quite similar), there's FireSass, which:


  allows Firebug to display the original Sass filename and line number of Sass-generated CSS styles.


There's nothing for Chrome at the moment (see Less/Sass debugging in Chrome Dev Tools/Firebug)