DNS assignment is done through DHCP. If your controller is giving out DHCP addresses, then check the scope:
show running-config | begin "ip dhcp pool"
Look for the dns-server entry for the respective scope.
ip dhcp pool vlan10
default-router 10.10.10.1
dns-server 4.2.2.2 4.2.2.1
domain-name domain.com
lease 0 6 0 0
network 10.10.10.0 255.255.255.0
authoritative