I think that you could pay the necromancer to "identify" corpses for you. So you pay money upfront before you even know whether the parts contain skills or not.
Without the necromancer every body part you find should look the same, without categorization.
To be honest, without categorization, here's how things will work:
The player will save their game, test all the body parts, and learn which ones have skills, then load and organize their inventory to put the skill ones together. The rest will be eaten immediately. (On my first playthrough, I used to save the body parts in case I needed to heal later, but a long time into the game I realized that I was never needing to heal by corpse-eating. So you might as well eat all non-skill parts immediately.)
Then they would have the necromancer only identify the parts they've already set aside or hadn't eaten.
That heavily depends on the skill. For some character builds it's pretty impossible to gain certain skills because they'd require huge ability costs. So getting skills by different means - circumventing ability requirements! - could be a very valuable trade, even if you have to give up something of value yourself in order to gain it.
What ability requirements or huge ability costs?
Things don't work the same way they did in D:OS 1. You only seem to need one ability point invested to use a skill, you get 1 ability point a level, and each increase in ability rank costs 1 ability point, but the maximum ability rank is at least twice as high, maybe 10 or even 15 points.
(And besides, I thought that the point was to get a skillbook from a body part so anyone could use it, and wouldn't those skillbooks have the same requirements as store-bought ones?)