Usually with a spark plug gap tester. "Fancy" isn't always better - a basic tester, or removing spark plug / checking against engine ground will allow you to see the 'quality' of the spark (bright/blue, vs. dull/yellow), an indicator of ignition wire & coil health.
re: "how do I fix code P0300?" - well, P0300 is a "generic" misfire code, and can be triggered by several different causes, including ignition system issue(s), fuel delivery or injector issue, or air delivery (vacuum / leak) issue.
Before "throwing parts" at the DTC (issue), best to do some diagnostic to narrow down / identify the actual issue.
Here's a YT video of some different spark plug gap testers, all will allow to "see" if the coil is firing, a few will also show "quality" of the spark.
