The way the CD lines work, they close a switch when a card is inserted. The software is supposed to check to see if the pin is open or closed. So you could connect both together and bring that to an input pin, and maybe the library you use checks for it, or maybe it doesn't. I think the better libraries will do read test from the card to see if its responding correctly.
If you have one of the sockets, you could put a meter on the pin and Gnd on the socket, see if putting a card on closes the switch(es) or not.
Sometimes one of the pins is Write Protect, you could try that out too, see if putting some tape or similar over the write protect notch (or whatever uSD/SD cards use) and see there is any change in switch status.