A magneto has a very low impedance stator winding inside. If the points are closed, the meter will see 0 ohms and depending on what model the magneto is, you might only see 1 to 3 ohms when they're open. It isn't like a distributor. So, you can't use a typical "continuity tester" type of buzz box easily.
You could watch the impedance change and see when the points are opening.
We carry an inductive buzz box that doesn't have this problem...it's smart enough to know what's going on and indicated OPEN and CLOSED on the front.
Definitely, better set it before firing...
Spud