easiest way to plumb a tapered flagpole?

Almost right Jellicoe. Use 2 spirit levels but diametrically opposite each other until the offset of the bubbles is the same, then move them round 90 degrees to get another reading.

When I put mine up I was able to attach a plumb line near the top attached to the bracket that the flag pulley was attached to. The bracket extended farther than the poles taper.

When done I just pulled to release the plumb line at the bracket.

The hardest part of doing it this way is waiting for a time period when there is no wind to mess with the plumb line.

Using a level shimmed to compensate for the taper might be a better way.

Use a wedge to compensate for the taper. Then you can use a carpenter's level.

Use two spirit levels more or less at right angles to each other and obtain the same amount of bubble displacment in each. The pole will then be plumb.

Dig a straight hole and measure equally in from the sides.