Sascha,
There is not a a way to directly check wether or not the terminal is connected. If you take a look at the file open VI you will notice that the default is and presumably there is a case statement which checks for this value and pops up a file dialog if it finds it.
Perhapse you may want to approach the problem from an other angel. Maybe making the VI take a boolean which will change its behaviour from the wired to the unwired mode.
Hope this helps!
Regards,
Amaury R.
Applications Engineer