It's not easy comparing different types and shapes of magnet. The magnet on its own is not very important, what matters is the flux you get in the gap of the intended alternator. This is very much affected by the magnetic circuit that you put round the magnet.
All you can do is to compare identical size and shape magnets by using "pull" or similar things.
If you can determine the type of material it is made from and can find where the poles are then you can use maths to work out the flux you can get in a gap. Even if you have a fluxmeter the reading on a magnet alone with no magnetic circuit tells you very little and you still need to do the maths for the circuit or build an iron circuit and use the fluxmeter to measure gap flux. If you go this far then you can use a search coil ( test coil) and spin the thing at known speed and deduce the flux from the volts produced.
The old magnet steels are little use for wind power. You should be able to roughly identify ceramics by looking at them but there is significant difference between the grades. If the thing seems far stronger than ceramic then it is likely to be neo or Samarium Cobalt. Unless it was military or from aerospace or something the cost of Samarium cobalt makes it a bit unlikely but neo has not been around long so if it is an old device it is again unlikely.
Flux
