I have Steam running on Ubuntu and loaded using 'optirun'. Will it follow that games that are started through Steam will also be using the Nvidia discrete card? It doesn't seem to look that way.
What commands can be used to tell that it's running on Nvidia?
No, they will not be using nvidia's discrete card. You're not alone in trying to find a solution for this, I'm currently trying to figure out a way that doesn't involve running
LD_LIBRARY_PATH=~/Steam/ubuntu12_32 optirun ~/Steam/SteamApps/common/$GAMEYOUWANTTOPLAY/$NAMEOFTHEIRBINARYFILE
in a terminal every time you want to run a game. On the plus side, it does mean you don't need to optirun steam at all, invoking that launch command will run the game itself with the discrete card regardless of what steam is running with.
Yay! Steam posted an official support page for this. https://support.steampowered.com/kb_article.php?ref=6316-GJKC-7437
I haven't tried it yet but hopefully it works.