You can not detect a card or device that is monitor mode and only sniffing passively. It accepts data in but no packets are sent out.
If the attacker starts injecting packets on the same card to crack your wep encryption that is detectable. A wireless IDS could detect this activity i believe.
The PDF at the link below has a small section entitled "Wireless Attacks Detection Techniques".
How come your laptop can't connect using WPA?