You can register a function that will be called each time a variable is modified by calling UNotifyChange, passing either an UVar or a variable name as first argument, and a member function of your UObject as second argument (and optionlly a String array containing the name of the types of the parameters). The prototype for UNotifyChange is:

void UNotifyChange (String var_name, String method_name, String[] args_name)
void UNotifyChange (String var_name, String method_name)
void UNotifyChange (UVar v, String method_name, String[] args_name)
void UNotifyChange (UVar v, String method_name)

The callback function can take zero or one argument: an UVar pointing to the UVar being modified. And the callback function must return an int. The notifyChange callback function is always called after the variable value is changed.