from qgis.core import *
from qgis.utils import *
from PyQt4.QtCore import *
from PyQt4.QtGui import *

devices_combo_box = qgis.utils.iface.mainWindow().findChild(QComboBox, 'mCboDevices')
com_port = devices_combo_box.itemData(devices_combo_box.currentIndex()).toString()
detector = QgsGPSDetector(com_port)
QObject.connect(detector, SIGNAL("detected( QgsGPSConnection )"), self.connection_detected)
QObject.connect(detector, SIGNAL("detectionFailed()"), self.connection_detection_failed)
detector.advance()

def connection_detected(self, gps_connection):
    QMessageBox.information(None, "Information", "Connection detected", QMessageBox.Ok)

def connection_detection_failed(self):
    QMessageBox.information(None, "Information", "Connection detection failed", QMessageBox.Ok)