The only way to tell really is to contact the producer and the mastering house and ask whether or not the vinyl came from an analog master.
But I wouldn't fear digital...if the vinyl came from a high resolution 24-bit digital master then there's no reason why it still won't provide a better experience than the drc'd clipping CD.
Still CD when done properly can yield good results - take a look at Chinese Democracy, no clipping, no dynamic range compression.
As far as Nevermind is concerned you might want to try looking at a different pressing. If you're listening to the Back In Black I'd suggest going to Discogs and trying to pick up an original Geffen pressing, those usually seem to be the best. The Simply Vinyl, Japanese, MOFI, Universal, etc., pressings don't seem to be as good sonically as the original pressings.
European Geffen pressing - 9 for sale -
http://www.discogs.com/Nirvana-Nevermind/release/380426
Some studios can be pretty good with communication. I picked up the new DMB album on vinyl and my ears were telling me it was sonically superior to the CD. I e-mailed Ted Jensen's project manager at Sterling Sound and within an hour they e-mailed me back to confirm that the vinyl was a 24-bit master.
You might enjoy reading Tom Port's site -
http://www.dccblowout.com/dept.asp?dept_id=14-006-013
He has some interesting vinyl theories. He also sells "hot stampers" and grades them - you'll pay good money but it's a money back guarantee. I haven't gotten any yet but a friend on the blu-ray forums has and he's elated with the hot stampers.