Yep, once you have installed Half-Life / Counter strike v1.6, and installed Earth Special Forces, you can play offline. Simple disconnect the internet, and connect to steam. Steam will detect that it can't connect to the internet, so it will go in offline mode. Once in offline mode, you can launch Earth Special Forces and play offline with bots if you wish.
But sometimes with me, i'm not sure if i'm the only one, then I can't play offline, it's some error, oh and remember you have to be logged in the steam acount before playing offline, otherwise they say something about "steam can't connect into offline mode, because there is no steam log in information on this computer", or something like that.