Actually the new version Ettercap has some nice detection features in it for promiscuous detection.

ifconfig eth0 -promisc : doesn't do anything I donno why?
Hrmm.. works for me. Are you doing this as the root user? Which distro is it (perhaps there is a bug in their version)? Do you have libpcap installed? Typing ifconfig should identify if it's promiscuous or not.

promiscuous
eth0 Link encap:Ethernet HWaddr 00:D0:59:GG:GG:GG
inet addr:192.168.0.5 Bcast:192.168.0.255 Mask:255.255.255.0
UP BROADCAST NOTRAILERS RUNNING PROMISC MULTICAST MTU:1500 Metric:1
RX packets:37721 errors:0 dropped:0 overruns:0 frame:0
TX packets:43512 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:14575098 (13.8 Mb) TX bytes:7317397 (6.9 Mb)
Interrupt:10 Base address:0x4000
non-promiscuous
eth0 Link encap:Ethernet HWaddr 00:D0:59:GG:GG:GG
inet addr:192.168.0.5 Bcast:192.168.0.255 Mask:255.255.255.0
UP BROADCAST NOTRAILERS RUNNING MULTICAST MTU:1500 Metric:1
RX packets:37731 errors:0 dropped:0 overruns:0 frame:0
TX packets:43524 errors:0 dropped:0 overruns:0 carrier:0
collisions:0 txqueuelen:1000
RX bytes:14575926 (13.9 Mb) TX bytes:7318255 (6.9 Mb)
Interrupt:10 Base address:0x4000