PC's even dead ones are a wealth of parts. I was out at a site and they asked me to do some troubleshooting, but would you not know it, I did not have my tools with me. They did have a dead pc about to be trashed. so I borrowed a part or two to make the tester.
Note: This setup may not work well with very long wires.

Step 1: Gather the Parts.

You need:
    Bios battery holder removed from a dead motherboard.
    Led from front panel of computer
    Two wires from led to motherboard separated.
     Bios battery (working).

Battery holder can be easily desoldered by melting just two points. (battery must not be installed)
The led and wires should be easy to dismantle.
Battery should be good, if not they are cheap.

Step 2: Solder It Together.

This might be a tricky part. Led's only work one way. You will need to hook the led directly to the battery holder to see which end or side to use. (Hint: battery holder and battery can be used as an led tester) The two solderable points on the battery connector look different. I bent the end of one side of the led to remember when end to solder. With the battery NOT in the holder solder a wire to the end the led is not going to. Then solder the correct side of the led to the other connector. Lastly solder the other wire to the other (unconnected) side of the led. You can kind of crimp it together first to see if it works.

Step 3: Test It.

Put the battery in the holder. If they are not already, bare the other ends of the wires and touch together and the led should light. If it does not. recheck the battery to see if it is good and then recheck your solder connections. (I turned off the lights.) Mine worked. At this point you can add fancy alligator clips or test ends like a multimeter or even put it in a case, but it works fine as is to me.

Step 4: You Are All Done.

Now you can go test to see if a wire or whatever is a complete connection. Warning: Do not ever connect it to a live wire or circuit!!!! Use at your own risk. I will not be responsible for any issues. If you are unsure, get a qualified person to help. McGuyver would be proud....

Step 5: Late Night Messenger.

Add a doorbell button and you have a no noise late night message sender for using your made up code or even using Morse code. You need one unit for each of the people involved. A foil reflector (not shown) makes for a brighter light.