If you find yourself with an unfamilar SharePoint 2013 farm with a problem with Office Web Apps the first step you’ll probably take is to figure out where Office Web Apps is running. Finding this out is simple!
Open an elevated SharePoint Management Shell on one of the SharePoint servers in the farm with an account with SPShellAdmin rights and run the following cmdlet:
The output will list all the bindings by application/extension and zone. Find the combination that is causing you grief and look at the ServerName property. This is the public hostname used by Office Web Apps. If it’s a server name go connect to it. If it’s a DNS name that abstracts away a server name or load balancer you’ll need to do more digging but at least you’re heading in the right direction. 🙂