Personally for the 60 degree blind spot, I'd draw a straight line between the attacking ship and the targeted ship. If the line passes through the hex side directly behind the targeted ship, the attacking ship is in the blind spot. If it does not, then the attacking ship is not in the blind spot.
The fun part is when the line goes through a hex corner between the two. Do we use Ad Astra's rule about rounding in favor of blowing stuff up, so it would count as being in the blind spot?
For 120 degrees, you have two hex spines, one at the 4 o'clock to the targeted ship, and one at the 8 o'clock. I am not sure about the hex spines, but between the two spines is definitely in the blind spot.
But it sounds like the OP has a special reason for asking. I wonder what OP's angle really is.