Check for start_sessoin without causing header problems

Hello I checked the PHP manual and couldn't find anything on this... When I call session_register() an implicit call to session_start() is executed. So if the session handling uses cookies to track the session and the call to session_register() is not before any output this will cause a header allready sent problem. Is this correct? If so how can I call a register session from within a class and make sure that the session is allready active and if not just print a warning. For background information: The class mentioned above is part of a classlib used by others and I can not guarantee that they call session_start since I have no controll over how my class lib gets included. So the issue here is if it is possible to create a backup for this problem or if I just have to rely on the user of the classlib to understand the mechanisms of session tracking. Any ideas on this? What do you consider best practice in this matter? Thanks for all the advice I have received so far Stefan