You'll need some sort of code reader, or will need to take it somewhere
that has one. In many places, Auto Zone advertises it will read codes for
free. I'm guessing you have a code something like P030x where x represents
the misfiring cylinder. You can then check the plug, wire, etc. on that
cylinder. You can check them beforehand, but won't find out if there are
any other trouble codes which may indicate other relevant problems.