How do I know if the file is already checked out by someone else?

Some version control systems notify other users when you check out a file. Others let you lock a file, so that nobody else can check it out. Code Co-op doesn't do it for several reasons. First of all, it's not very practical in a distributed environment, where notification often travel by email. Second of all, there is no central server, so there is no sure way of locking a file, short of waiting for the lock acknowledgment from everybody. Finally, Code Co-op relies on conflict resolution rather than the more obtrusive conflict prevention. Which doesn't mean that in future releases we won't implement optional check-out notification, especially on local network systems.