As a general rule, if mining a tile of adamantine reveals an unrevealed tile within the pillar it is not worth the risk. If mining a tile of adamantine does not reveal an unrevealed tile, it's 100% safe.
Also note that the hollow parts are always 2x2 squares. If mining a square of adamantine reveals an unrevealed square that cannot be a part of a 2x2 hollow structure, it's again 100% safe. For example, something like this:
........
...****.
...*uu*.
...*u**.
..**u*..
..*uu*..
..*uu*..
..****..
........
. = mined tile
* = unmined adamantine tile
u = unrevealed space
If a z-level of the vein looked like this, the entire top half is safe to mine because while the inside is unrevealed it cannot form a 2x2 structure and thus cannot be the hollow part of the tube. The lower half, however, could be hollow, if the hollow part of the tube extended this high.
Edit: Whenever I discover cotton candy, I do this, in order:
1. Mine out the entire area around the vein, without touching the adamantine. Go up and down the whole spire as far as possible. Magma sea usually makes the bottom difficult, but you can usually nab some free tiles down there.
2. Determine which tiles are safe and which are risky using the above. Designate all safe tiles to be mined.
3. Have my engravers smooth the entire risky part of the spire. Then, starting from the top, having them carve fortifications into the rock one level at a time. This will reveal the insides of the spire; if it is solid, mine it out. If it is hollow, you will trigger the demons, but they will not have a path into your fort. Your FPS will drop, but it'll return eventually. Have your engravers run away, seal that level off and never go there again. This will also reveal the entire hollow part of the tube, letting you know which tiles are safe to mine below your reveal point; might as well mine those.
4. Profit!