Before beginning it is very important that you note that the output (secondary) side of a Buzz Coil is extremely high voltage and is very dangerous. That voltage is going to be exposed on "open to fingers" terminals in this article. Handle it the way you would handle a live spark plug!
Before testing your coil, clean it. Then hook it up to the tester as shown in the tester article.
With the toggle switch off, the buzz coil should not operate, and the Voltmeter and Ammeter should show zero.
Throw the toggle switch, the buzz coil should start to buzz and there should be a spark jumping across the spark gap. If you are running at 6VDC, the Ammeter should be reading between one and two Amps. If you are running at 12VDC, the Ammeter should be reading between half and one Amp. Note that if the coil is buzzing, but there is no spark, your coil may only need adjusting or it may have a failed coil internally. The Ammeter may read in either direction (ie. it can swing left or right).
While the switch is on, there should be a constant "stream" of sparks leaping the spark gap. If this stream is not constant the points are probably pitted and should be cleaned with 600 grit sandpaper or an oil stone. In extreme cases a fine jewelers file can be used. Before cleaning the points try turning the adjustment screw on the top of the coil one way then the other to see if the sparking improves. The adjustment screw is live with the battery voltage, but should not be live with the spark voltage and so should be safe to touch.
If the current is below 0.6 Amperes at 12VDC or below 1.2 Amperes at 6VDC then the vibrator spring is probably weak. Remove the coil from the tester. Remove the contact spring and gently bend the vibrator spring upwards until there is a 7mm (1/4") gap between it and the core when it is sitting in a neutral position. If the gap is already 7mm, try slightly increasing it.
If the current is above 0.9 Amperes at 12VDC or above 1.8 Amperes at 6VDC then the vibrator spring is probably too strong and stands too far away from the core. Follow the same procedure as in the previous step except you should bend the spring down to reduce the gap.
If the Ammeter reaches the end of its scale this indicates the points are stuck together and possibly fused. Remove the coil and clean the points. If the points are OK, and the mechanism is free to move then the coil should be replaced or serviced.
If it is not possible to get a good hot spark at less than 0.75 Amperes at 12VDC or 1.5 Amperes at 6VDC then the coil is marginal. For a stationary engine operating from a battery you can keep using it (it will just flatten the battery faster) but if you are using a low tension magneto to power the coil, then discard or service the coil. A low tension magneto will not generate enough current to run the coil at the low revolutions that most of our engines are operated at.
What can you do at a show if you suspect your coil is bad, but do not have your tester with you? Simple enough, remove the spark plug from the stopped engine and lay it on the engine someplace where it will make a good electrical ground contact. If you have a compression relief valve, use it, it will make this next job easier. If your engine is multi cylinder disconnect the power to the other coil(s) or ground the spark wire(s) - you do NOT want the engine to start running during this test!!
Crank the engine and watch the spark, if it is nice and hot and blue chances are your coil is OK. Note that if you do not make a good electrical ground contact with the plug guess who becomes that path - yep you, ZAP! If you want to test the coil further, remove the lead from the spark plug and holding it in an insulated holder (eg. wood or plastic) crank the engine while holding the wire 7mm (1/4") from the engine body. This time get a friend to crank the engine because if you are not close enough, again it will zap the crankee!