HOW TO TEST GAMES FOR STEAM CLIENT DRM
Checking Steam games for DRM is a simple process: Just rename your main Steam program file and try to run a game. If the game runs, then it's DRM-free,
I don't think it has been mentioned, I skimmed through the thread rapidly, sorry if I miss it, but this is not always true. some games, especially the earlier ones that used Securom, can be run with Steam totally uninstalled but that doesn't make them DRM/free; because if you try to copy them to another computer and run them there Securom will ask you to reactivate the game online.
This is also especially true for some of the early EA Origin games, for example Dead Space 1 was like that (maybe it has been patched since and maybe also Crysis 1 but I am not sure); you could download and install the game through Origin, then uninstall Origin and the game was perfectly playable.
But if you tried to copy the game to another computer or modify your hardware then you will end up with Securom pop-up asking you to reactivate it.
IMHO the only bullet proof way to test if a Steam/Origin/Uplay is "DRM-Free" is to copy it to another offline computer/VM on which it never was installed and try to run it.