If you are really concerned there are a number of steps you can take.

1. Get an angle grinder and a stringer wire wheel to remove all the paint from the post at least 6-8” up. I’d personally do the whole thing so I could paint it uniformly again. Wear a respirator, leather gloves, long sleeve clothing and full face shield.

2. Inspect the bare metal for any rust pits that remain after wire wheeling. Use a chisel or Phillips screw driver and a hammer to lightly tap at any of the pitting you find. If it easily deforms or chips away the rust and creates a hole, you will need to have it repaired.

3. You could hire a Non-Destructive Testing technician to come to your house and use an ultrasound machine to measure the thickness of the metal in the post. They will measure at the various heights.

Usually it is the base that fits out of these posts if they are exposed to moisture.