The browser will open a separate connection to get the style file. So you cannot directly communicate with the script that included the style. But using the parameter is an easy way to distinguish between scripts.
I also experimented with storing in the database some value from the script accessed and then check it in the style file. Depends what you want to do, I found it useful for some statistics. But I did not find it useful to distinguish between visitors and bots.
The extra parameter with the link tag is the easiest and most effective though.