Better to use the ground cable rather than positlve. That way if you touch something it won't short something, "like your wrench"
You make certain everything is shut off, check for things like dome, map, trunk lamps.
Remove the battery ground, and hook one end of your test lamp to a good ground. Stab the probe of the test lamp into the battery NEG post. If it lights you have a drain. Look carefully in dim light, because a "slight" drain may barely light the lamp, or not at all.
Really. And I mean REALLY, if you have one of these old girls, you should spend 30 bucks or so and buy yourself a multimeter and a bag of clip leads from Radio Shack. Make sure the meter you buy has some DC current scales, and can read current up to at least 10 amps.
After you check with a light and determine that it does NOT light, now it's safe to check with a multimeter. Same deal as the lamp. Set the meter up for DC current, and start with the largest scale, 10 amps, 20 amps whatever. If you do not get a reading, set the meter up for less current and recheck.
If you post a meter you are interested in, that is the model and brand, we can guide you as to whether it's got the appropriate scales