Telling the type of a pointer

Hi,

if I have an object attached -- via object_subscribe() or object_attach() -- to several different types of sources, how can I identify the type of the notifier object? Let's say, I register my object to a t_dictionary and also an own-developed custom object. When I receive a notification, how can I tell the type of the sender argument (typed as void *) of my notify() function?

Thanks,
Ádám