How can I determine if a lag issue is due to my own hardware or my connection to the ISP?

I have dealt with intermittent lag spikes for years. It's not a big deal for most things, but I get kicked from competitive FPS matches on my PC and get separated from my wife when we play GTA on our consoles. We have called our ISP multiple times about it. At one point, they sent out a technician who seemed unusually frank and helpful based on our past experience, and he said (this was a few years ago, and my memory of it is inexact) that the connection from our house to the nearest whatever (access point?) was degraded and needed to be replaced. After that, I feel like I was gaslit a bit, as

Question by Dr.Queso on Super User, licensed CC BY-SA. We show a short excerpt and link to the original thread as credit.