Really...it's nearly impossible to install shocks wrong.
Not installed, maybe.
But it's just two bolts...two bolts that hold it in place, and without either bolt the shock would flop around and you'd hear it for sure.
Crawl under and simply grab a shock with your hand and see if you can move it.
If either bolt is missing, it will move around easily.
If it won't budge, it's bolted in.
Bilsteins have a lifetime warranty.
They don't take long to remove/replace, especially the back ones.
Why not pull them out and get them checked out?
Or...perhaps they didn't install the correct shock?