No mag drop on runup is UNACCEPTABLE. You need to know both

You can wait until it gets dark. Pull the airplane by a mercury vapor street
light. Try to set the RPM at 1800. If the prop appears to stop at 1800 RPM your
tach is right on the money.

Run the engine until the prop appears to stop like if a timing light was
shining on it.Do your mag check. The prop should appear to turn to the left
because the RPM dropped. If it does this you should be able to tell if the mags
are independent of each other.

This takes some precision throttle movements to get the initial 1800 RPM
setting, but in the end the whole process is self explanatory.

This is also a tach check. The prop should show some motion change at 600 -
1200 - 1800 - 2400 RPM. Three blade props might or might not work like this,
logic says they will but at a different RPM.

Hope this helps.

Just My 2¢ Worth