i want to Update my pair of 5520 ASAs.
What are the best functionality tests of application/routes to check if certain traffic goes over the FW.
To check it in theory, use packet-tracer.
To check in it with actual live data, do captures on the input and output.
take your most crucial apps and/or user traffic and test that after upgrade. not sure if you are hosting www behind asa, maybe in conjunction with a load balancer, but i would check that. possible remote VPN.
no silver bullet test i am afraid