A: Simple: install the game and launch it once (this step is important, because this finalises the installation). Shut down Steam. Locate game folder, locate the .exe, run it. Either Steam is launched and your game right afterwards, or the game just runs. If it just runs, it is DRM-free.
If you want to be 100% sure a game is really DRM-free the only sure test is :
Copy the game folder to another offline
computer, partition, VM, whatever, on which Steam was never installed and try to start the game there. If it start and runs Ok then the game is DRM-free.
I have some game that worked fine "without" Steam started, but refused to start on without Steam installed and/or without the local user registration information (the stuff that allows you to run Steam in offline mode). Also you have games with third partied DRM that might accept to run without Steam but are far from DRM-free.