In iChat version on Snow leopard I could view all the previous chat messages of a user. But since the current version (iChat 7.0.1 Mountain Lion) I cannot see that chat History. All my previous messages are saved into ~/Documents/iChat by previous version. So how can I view the chat history like in previous versions?
For chat transcripts created by Messages, once you've selected the conversation in the Messages window (with File > Recent Items if necessary), you just scroll back and Messages will load older transcripts incrementally. However, this is pretty poorly implemented, as there's no way to scroll all the way back at once; you have to scroll to the top or press Home repeatedly.
The newer transcripts are stored in
~/Documents/iChat. Quick Look is also supported for chat transcripts.
Chatology is a third-party app that may be of help; it will read transcripts in both locations, is faster and less buggy than Messages (e.g., lets you see images in old chats where Messages won't).
You can open old chat transcripts either by double-clicking the .ichat file in ~/Documents/iChat or by using Cmd-O from within Messenger. Both options aren't really convenient...