if you've done a bit of auto electric testing.. then a test lamp should be in your tool box.
it's basically a probe and lamp ad gator clip. clip cator clip to ground usually..a nd then use the probe to find hot.. esentially.
the good ones will work on 6v or 12v. IE.. glow dim on 6 and glow bright on 12...
you don't need that tool.. however. any suitable voltage bulb and wires works fine.
you can make a cheap test lamp with a trailer marker lamp.. 1$ from the flea market.. etc. they have a 168 or 194 bulb in them usually..