passive interface, eigrp and ospf...

this command can certainly be a pain in the ass...

ospf gives you a decent way to detect it with:

R1#sh ip ospf int s1/0.12
Serial1/0.12 is up, line protocol is up
  Internet Address 10.1.1.1/30, Area 12, Attached via Network Statement
  Process ID 1, Router ID 1.1.1.1, Network Type POINT_TO_POINT, Cost: 64
  Topology-MTID    Cost    Disabled    Shutdown      Topology Name
        0           64        no          no            Base
  Transmit Delay is 1 sec, State POINT_TO_POINT
  Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
    oob-resync timeout 40
    No Hellos (Passive interface)
  Supports Link-local Signaling (LLS)
  Cisco NSF helper support enabled
  IETF NSF helper support enabled
  Index 2/2, flood queue length 0
  Next 0x0(0)/0x0(0)
  Last flood scan length is 1, maximum is 2
  Last flood scan time is 0 msec, maximum is 12 msec
  Neighbor Count is 0, Adjacent neighbor count is 0
  Suppress hello for 0 neighbor(s)
  Message digest authentication enabled
    Youngest key id is 1

although you have to look; it doesn't jump out at you...

eigrp isn't as friendly...

R4#sh ip eigrp int
EIGRP-IPv4 Interfaces for AS(10)
                              Xmit Queue   PeerQ        Mean   Pacing Time   Multicast    Pending
Interface              Peers  Un/Reliable  Un/Reliable  SRTT   Un/Reliable   Flow Timer   Routes
Fa0/1                    1        0/0       0/0          19       0/0           84           0
R4#config t
Enter configuration commands, one per line.  End with CNTL/Z.
R4(config)#router eigrp 10
R4(config-router)#no passive-int f0/0
R4(config-router)#
*Feb  9 09:45:59.609: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 10.1.4.6 (FastEthernet0/0) is up: new adjacency
R4(config-router)#do sh ip eigrp int
EIGRP-IPv4 Interfaces for AS(10)
                              Xmit Queue   PeerQ        Mean   Pacing Time   Multicast    Pending
Interface              Peers  Un/Reliable  Un/Reliable  SRTT   Un/Reliable   Flow Timer   Routes
Fa0/1                    1        0/0       0/0          19       0/0           76           0
Fa0/0                    1        0/0       0/0          19       0/0           76           0
R4(config-router)#do sh ip eigrp neigh
EIGRP-IPv4 Neighbors for AS(10)
H   Address                 Interface              Hold Uptime   SRTT   RTO  Q  Seq
                                                   (sec)         (ms)       Cnt Num
0   10.1.4.6                Fa0/0                    11 00:01:17   19   114  0  814
1   10.1.4.10               Fa0/1                    14 00:28:05   19   114  0  788
R4(config-router)#router eigrp 10
R4(config-router)#passive-int f0/0
R4(config-router)#
*Feb  9 09:47:35.717: %DUAL-5-NBRCHANGE: EIGRP-IPv4 10: Neighbor 10.1.4.6 (FastEthernet0/0) is down: interface passive
R4(config-router)#do sh ip eigrp neigh
EIGRP-IPv4 Neighbors for AS(10)
H   Address                 Interface              Hold Uptime   SRTT   RTO  Q  Seq
                                                   (sec)         (ms)       Cnt Num
1   10.1.4.10               Fa0/1                    11 00:28:32   18   108  0  788

the eigrp commands just show you that it's NOT there... debug doesn't help either...