ferrite (ceramic) magnets are made by mixing magnetic fragments into a kind of pottery clay which is then fired. they're not made of metal and so feel softer. they'll have a bit of the same feel as a rubbery fridge magnet--but obviously not quite as soft.
however, in my experience, the slug ceramic magnets used in single coil pickups are usually coated with something like chrome to give them a more metallic look. so, that rubbery feel will be more apparent on the humbucker magnets.
if you feel an alnico magnet, it will actually FEEL like metal. usually it has a rough feel due to the cross-hatch pattern that you see in the pix that wesman posted a link for.
that's all the info i can offer. maybe someone out there has a more scientific method for testing?