Well I think the Numenera corebook addresses this in a couple of ways (...and remember this is only for actions where the NPC's are not opposing the PC's in some way since they would otherwise be using their level) but then leaves the desired implementation up to the GM...
The first way is for the GM to just decide success or failure for an NPC based on his own desires, flow of the game, etc. This is the simplest method and I would use this for unimportant/irrelevant/scene dressing NPC's as well as for things that happen "off-screen" or outside the influence of the PC's and have no direct effect on them.
Another method touched on in the book is for the NPC's level to be assessed and compared with the level of what he or she is trying to accomplish and they either succeed because they have a higher level than the challenge or fail because they have a lower level than the challenge. I'd probably use this method for minor NPC's since it would allow them to tackle somethings when working with the PC's but still leave the major stuff for the PC's to handle.
Another (and I believe final method, though I could be wrong) is suggested for NPC vs. NPC (but I plan to use it in a more widespread way for Major NPC's). This method is for the NPC's to be treated like a player with rolls, etc. This method, IMO, is a little imprecise since PC's and NPC's don't work on the same system, and NPC's don't have the stats (training, specializations, etc.) that are used in rolls. The book is kind of vague on how to address the fact that most NPC's only have a level.
I've been thinking about refining and adding to this by deciding that, when using this method, an NPC's level is subtracted from the difficulty level of a particular task before rolling to succeed or fail. So if the level of a task is 3 and a level 1 NPC is attempting it... it would be a level 2 task, roll over 6 while a level 3 NPC would automatically succeed. This modification draws heavily from the second method (but instead of making it an absolute pass/fail instead gives a chance for the NPC to succeed even at difficulties of a higher level). This is just something I thought of while reading the book and I would still need to see how it is in actual play but I think it should work pretty well since NPC's can't use effort... though I'm not sure if I would allow them to use assets such as tools (though I'm leaning towards yes).