You can use the universal transformer equation to work out the flux density in the core, ferrite is safe around 100mT, some cores can take 250mT.
By winding 10 turns and measuring the inductance you can caluculate the al factor, you can also use this as your test winding, you'll need a high freq, 30 - 40 khz.
If you scroll half way down this page you'll see an inductor test bench, I have made one of these, most usefull, I put the sense resisitor in the ground leg o fthe fet, so my 'scope ground is at ground level.
**broken link removed**