The positive feed to the coil is constant, the coil is always connected to the key switch output circuits.
However, the negative side of the coil is controlled by contact breaker points (if old school type distributor) or an ignition module (if modern style distributor.
You can only detect a negative circuit at the ignition coil when the breaker points, or ignition module are giving the coil a negative path to ground.
If the contact points or ignition module are functioning properly, you should be able to detect a "pulsing" negative path (using continuity tester) on the distributor wire while cranking the engine. The negative wire must be disconnected from the coil to do this test.
An alternative test for the same is to hold a test light to the negative side of coil (negative wire connected to the coil for this test) and crank the engine. If the test light dims (or goes dark) and brightens in a series of uniform pulses while cranking, it may be presumed that the distributor primary circuit is working right.
If the test light remains brightly lit throughout the test, there is a problem in the distributor primary circuit. The primary circuit of a distributor handles the task of controlling the negative side of the coil to produce, and time....the ignition spark.
The spark occurs when the primary circuit OPENS.