Typically you can call the slots that signal is connected to as long as the slot routines are not private.
[edit:koahnig] should checked before.
in the description of signals and slots , you can find:
Signals are emitted by an object when its internal state has changed in some way that might be interesting to the object's client or owner. Signals are public access functions and can be emitted from anywhere, but we recommend to only emit them from the class that defines the signal and its subclasses.*