Santa Cruz Techies

This part is intended for people with higher technical background.

There are two main methods we're using to detect ISP and wifi issues:
1) Connect to the modem/router itself, and evaluate the values the modem gets from the ISP.
Signal levels and uncorrectable errors on the modem/router might indicate that something is going on with the internet (AKA ISP)
2) Test the internet with an ethernet cable connected directly from the computer to the modem/router, and compare it with the same tests over wifi

Speedtest.net is our favorite website to run speed tests. We found that many websites provide inaccurate information about your internet speed.

For example, let's say that you ran a speed test multiple times. You got a download speed of 270Mbps to 330Mbps with your computer connected via ethernet cable to the modem/router.
 
Consistent results (up to 10-30% change between test results) from one speed test to another are good signs that your internet (ISP) might works fine.

On the other hand, if you run speed tests multiple times (via ethernet cable), and three times you get 270Mbps, two times you get 70Mbps, and another five times you get 600Mbps, it's a sign that you might have an internet (ISP) issue.

Similar tests can be done via wifi. 
When running multiple speed tests over wifi and getting extremely different results, it's a good indication that something might be wrong with your wifi (assuming that you have excluded the ISP as the main cause for the problem).