Speedtest by Ookla is the most widely used tool for measuring internet speed. You click a button, and it tests your download and upload speeds along with latency and jitter. It automatically picks a nearby server, though you can choose a different one manually if you want to test against a specific location. The results give you a quick sense of whether you're getting the speed you pay for, and whether a problem is on your end or your provider's.

There's a free web version and mobile apps for iOS and Android. You can create an account to save your history, which is helpful if you're documenting repeated issues with your connection. It's not a deep diagnostic tool, but for everyday troubleshooting, it's the standard.