The best approach is to baseline your web performance from sites with known good connectivity. Once you have that established, you can then begin the fairly straightforward process of establishing the cause of latency for your particular client <--> web application issue from the client in question.
My recommendations for baselining latency would be pingdom.com - $10/month gets you rock solid (I've been using them for the last six months - every issue they've reported has been real) latency analysis from multiple locations on the Internet as well as monitoring/notification for website down issues.
Of particular importance to your requirements, is that they establish an audit log of your web sites performance from multiple locations in the United States, as well as internationally. This can prove useful if you have a transient issue and you wish to look back historically.
Being able to provide your boss (who is your target audience in the question) a graph of average latency from twelve different locations provides a strong foundation for identifying the client-side issues (WiFi, DNS, laptop problems) versus an issue with the website's base performance AND connection to the Internet.
DNS server issues, of course, can be immediately remedied by having the client place the appropriate hostname->IP mappings in the local /etc/hosts (or, in the case of Windows, C:\windows\system32\drivers\etc\hosts) file - If the issue disappears when you have the appropriate /etc/hosts file configured, it's a very strong indicator that there is an issue in your resolver chain (local resolver, first DNS server in the chain, or canonical)