If you bring up the command prompt and type ipconfig /all then press enter you should see the primary DNS server in use
e.g. part of cmd prompt result
Windows IP Configuration
Host Name . . . . . . . . . . . . : xxxxxxxx
Primary Dns Suffix . . . . . . . :
Node Type . . . . . . . . . . . . : Broadcast
IP Routing Enabled. . . . . . . . : No
WINS Proxy Enabled. . . . . . . . : No
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Broadcom 440x 10/100 Integrated Controller
Physical Address. . . . . . . . . : xxxxxxxxxxxxxxxxx
Dhcp Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
IP Address. . . . . . . . . . . . : xxxxxxxxxxx
Subnet Mask . . . . . . . . . . . : xxxxxxxxxxxxx
Default Gateway . . . . . . . . . : xxxxxxxxxxx
DHCP Server . . . . . . . . . . . : xxxxxxxxxxx
DNS Servers . . . . . . . . . . . : 212.159.13.49
156.154.70.1
208.67.222.222
Edit > for me the primary DNS in use 212.159.13.49 is Plusnet
Microsoft Windows [Version 6.1.7601]
Copyright (c) 2009 Microsoft Corporation. All rights reserved.
Primary Dns Suffix . . . . . . . :
Node Type . . . . . . . . . . . . : Hybrid
IP Routing Enabled. . . . . . . . : No
WINS Proxy Enabled. . . . . . . . : No
DNS Suffix Search List. . . . . . : lan
Ethernet adapter Local Area Connection:
Connection-specific DNS Suffix . : lan
Description . . . . . . . . . . . : VIA Rhine II Compatible Fast Ethernet Adapter
Physical Address. . . . . . . . . : xxxxxxxxxxxxxx
DHCP Enabled. . . . . . . . . . . : Yes
Autoconfiguration Enabled . . . . : Yes
Link-local IPv6 Address . . . . . : XXXXXXXXXXXXXXXX(Preferred)
IPv4 Address. . . . . . . . . . . : 192.168.1.64(Preferred)
Subnet Mask . . . . . . . . . . . : 255.255.255.0
Lease Obtained. . . . . . . . . . : 27 September 2012 08:41:38
Lease Expires . . . . . . . . . . : 28 September 2012 11:56:22
Default Gateway . . . . . . . . . : 192.168.1.254
DHCP Server . . . . . . . . . . . : 192.168.1.254
DHCPv6 IAID . . . . . . . . . . . : XXXXXXXXXX
DHCPv6 Client DUID. . . . . . . . :XXXXXXXXXXXX
DNS Servers . . . . . . . . . . . : 208.67.222.222 < I know these are OpenDNS
208.67.220.220
NetBIOS over Tcpip. . . . . . . . : Enabled
Tunnel adapter isatap.{6E79A6E4-9E0E-4F97-89B6-CF5F96530DAC}:
Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Microsoft ISATAP Adapter
Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes
Tunnel adapter Local Area Connection* 11:
Connection-specific DNS Suffix . :
Description . . . . . . . . . . . : Teredo Tunneling Pseudo-Interface
Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes
IPv6 Address. . . . . . . . . . . : XXXXXXXXXXXX(Preferred)
Link-local IPv6 Address . . . . . : XXXXXXXXXXXXXXXXX(Preferred)
Default Gateway . . . . . . . . . : ::
NetBIOS over Tcpip. . . . . . . . : Disabled
Tunnel adapter isatap.lan:
Media State . . . . . . . . . . . : Media disconnected
Connection-specific DNS Suffix . : lan
Description . . . . . . . . . . . : Microsoft ISATAP Adapter #2
Physical Address. . . . . . . . . : 00-00-00-00-00-00-00-E0
DHCP Enabled. . . . . . . . . . . : No
Autoconfiguration Enabled . . . . : Yes