There are only four distinct areas in which all problems can originate: Account provisioning (a.k.a. your phone's permissions), network issues, device malfunctions, and user error.
The standard way to deduce is to first determine location of issue. If issue happens in various locations (i.e. under different towers), and not in an isolated location, then it is NOT a network issue. Yes, in the event of a nationwide outage, the entire USA is considered the isolated location.
Secondly, a Verizon Tech Support rep (store reps will not have the tools or knowledge to properly investigate) will have to check the provisioning set up on your account, and make sure the provisioning is correctly relayed to the towers. Your account provisioning are the set of permissions used to determine the services to which you require access. The example would be if you have a 3G phone, you should not have 4G provisioning.
Thirdly, your device would then be investigated to determine if there is a software, firmware, or hardware malfunction. Please note that when I say software, I mean that as in a software bug resulting from a recent OTA update from HTC. The apps you install causing malfunction fall under the last category...
User error. I think you all can use some common sense here. If you do something to the phone, knowingly or not, that causes the malfunction. The most common user errors are app-related, but Google is trying to remedy this with recent updates to the Google Play Store.
Now I do not expect every Verizon Wireless rep to know this process subconsciously, but I hope that everyone here knowing this deduction process can use it to help a Verizon rep help you better.