I don’t know of any good home tests for live beetles other than long term observation. Powder post beetles can be dormant for years, and can continue to re-infest by laying eggs even in fully dried wood. You’ll know they’re still active when they start exiting the wood, leaving a clean exit hole with a small pile of dust below. If you’re willing to chance it, clean the blank really well, put the it in a plastic ziploc bag and leave it for at least a few months. If you come back and find dust in the bag, then there are still active beetles. If there’s no dust, then it may or may not be clean. The longer you let it sit without observing any new exit holes, the more confident you can be.
You can try to cut them out by first locating all the visible holes and then cutting back until they’re completely gone, and then a bit more for good measure. All sapwood anywhere on the piece should also be removed. This sometimes works for lumber, but isn’t really a practical option for a bowl blank.
To be safer, you can kill them by fumigating with chemicals, which is a nasty process, or by kiln drying. Fumigation is the preferred method for dealing with an infestation in finished work like antique furniture. It will sometimes change the color of the wood but not in most cases wil not otherwise damage the piece. Kiln drying will kill all beetles and larva provided that you get the core temperature of the wood to 140 degrees and hold for an hour.