Since this is a very generic question, let me try and point you in the right direction.
First and foremost, make sure that you have an up to date diagram of the network - both logical and physical.
Once you have the above information in place, looks at:
L2 Security - L2 Domain, VLANS, Port Security, etc
L3 Security - Traffic across WAN,
Routing Protocol Authentication, VPN, Remote Users, Internet Edge Access-Lists, Infrastructure ACl, Out of Band Network.
N/W Device Management - Telnet, SSHv1/v2, SNMPv3
L4-L7 - Caching, HTTP/HTTPS
Configuration Best Practices
Review Security Configuration Template
Schedule Meetings with Various Teams and understand their security policy and how they manage the devices/traffic.
And also refer the SAFE Blueprints, which will be come very handy during your assessment.
These are some of the few that comes to my mind. I hope it helps.