I am running Slackware Current, kernel 5.10.30, KDE 5.21.4. I have a microphone plugged into the front mini-RCA jack. Both System Settings > Audio and Pulse Audio > Input Devices show the Front Microphone, presumably enabled. How can I test to see if this mic is working?

hopefully you tried searching online for an answer, such as:

command-line method for testing the microphone on your Linux PC

from there i was able find a command to run in a terminal:

arecord -d 5 test-mic.wav
note: .5. appears to set length of recording

type in the command, press enter and start talking...
hopefully into a microphone of your choice

after the recording is done, enter the 'ls' command and you should see the test-mic file
open that file to see if you got audio...

if no sound...research you audio settings
good luck

Sound applications like Audacity have input level meters, you can speak into your microphone and adjust the level.

Nope. Didn't even think about a command line tool. Good suggestion!
I tried pavucontrol, kwave and something else, already installed in KDE5. Audacity is not installed. kwave setup was complicated enough that I wasn't sure I had all the knobs and dials set correctly. I was looking for something simple to operate that I didn't have to download. pandanuma's 'arecord' suggestion was good. However, the recorded test-mic.wav file didn't play anything and mplayer just reported "Audio device got stuck!" I ran pavucontrol > Recording while running arecord and it always showed "No applications are currently recording audio", athough that could be because arecord is not KDE/GUI. I think the config is correct, but I seem to recall that the mic-jack never did work on this motherboard (ASRock 970M Pro3), even though System Settings and pavucontrol show it connected.

I'll next try a USB mic. I know that worked in conjunction with a mic incorporated into a camera. I'll post results.

Just a thought: if you're using the 'front panel' connection that requires an in-case connection to headers on the motherboard, and (in many cases) proper enablement in the system firmware. Try connecting the microphone to the rear mic-in jack - and see if that works out (I have had generally bad experiences with the header/front-panel connectors on most cases - they're usually super cheap).

The easiest way, IME, to test what you're ultimately after is to try recording yourself in Audacity or some similar application, and then playing the file back. Or if you have a chat application that supports voice, and a willing friend/test subject, try doing a voice call that way (basically use the microphone in the context you want to use it in - it will either work or it won't).