varying difficulties of similar tasks

How does one vary the difficulty of tasks at different times?

For example, in a circumstance where a PC tries to see through a deceit, imagine these two situations:

1) a common villager tries to conceal an item on his person while being searched by a skilled thief.

2) a powerful fey creature disguises her appearance from the same thief while she lures him to follow her to his doom.

In the first case, the thief is so much more skilled and knowledgeable than the villager, that he should be able, in almost every case, to outthink the commoner and find the hidden item. But in the second case, the fey is the one with the edge.  She's MAGICAL. She ought, in nearly every case,  except in an unusual circumstance, fool the thief.

When the thief makes his discern realities check in each of these circumstances, should there be a modifier? If so, what? Or if not, how do I deal with the fact that the thief's chance of discovering the trickery was exactly the same in both situations, despite the obvious difference in situation?

You can increase the difficulty by having an extra opportunity for failure. In the case of the fey, it's using mind-affecting magics, illusions, glamours and supernatural charms to ensnare the thief. So before the thief can even suss out the truth and Discern Realities, he has to avoid falling prey to the mind-trap; i.e. Defy Danger using Wisdom (the danger being getting glamoured).

There are no 'opposed' tests in DW. Player's characters may aid or interfere, and they can Discern or Parley against each other, but NPCs are just narrative tools of the GM.

1) Is it interesting to the story to have the villiger spot the thief? Is the villiager a named NPC with a stake in the unfolding events? If no, just say Yes and roll with the fiction, no roll required. Otherwise its using tricks of the trade as written. Follow the options on a 7-9! Make as hard a move as you'd like (as GM) on a miss.

2) The thief should only make a Discern realities check if you think that the fiction is supporting that move. 'Do do it, do it', right?
Is the thief closely studying the situation or fey creature? Then sure, roll away. The answers they get will depend not only on the roll, but also what your prep demands. Be honest with the players. Maybe the fey creature will avoid deep appraisal, no matter what the roll.

So when the thief discerns realities on the fey creature when she offers them the jewel they've been coveting and rolls a 10+ and first asks the question: What should I be on the lookout for?

You don't have to tell them "The Fey is trying to lure you to your Doom" (which is a truthful answer), but instead: "This Fey is magically powerful and is using all her wiles to reassure you of your safety, She is likely trying to fool you."

As the conversation develops and the thief asks their other questions, be generous with the truth, the truth being that the fey is much more powerful than the thief and likely to fool them every time. Be expansive, its about the bigger picture.

There's no modifiers for difficulty. How difficult it is to achieve a particular result, mechanics-wise, is a choice made by the player during character creation, not the GM during play.

The best way to handle these two situations seems pretty simple to me. Discern realities is the go-to move if a PC eyeballs the fey creature in disguise. Says so right in the description. Meanwhile, if a skilled thief searches a common villager, just tell them where they find the item.

Okay, I'll play.

1. A villager with a concealed item is being searched by Bulgur the Thief. The GM calls for a Discern Realities roll and Bulgur gets a 10. He gets to pick 3:
• What happened here recently?
• What is about to happen?
• What should I be on the lookout for?
• What here is useful or valuable to me?
• Who’s really in control here?
• What here is not what it appears to be?

Bulgur, which do you choose?

2. A beautiful woman beckons for Bulgur to follow her to his doom. Bulgur, not the trusting sort examines her closely. He rolls a 10+.

Bulgur, what do you choose?

1) What happened here recently? What should I be on the lookout for? What is useful or valuable to me?

2) Who's really in control here? What here is not what it appears to be? What should I be on the lookout for?

Thanks, your answers have helped, especially noofy and archangel. But more discussion is always good!

1) What happened here recently? What should I be on the lookout for? What is useful or valuable to me?

Well, its obvious you interrupted some kind of shakedown. It's definitely the little urchin who stole your coin purse, but its not on him now. He's been roughed up pretty good too. Keeps blubbering "don't hit me!" If I were you, I'd be careful leaving the place. You're all alone here and anyone who's been in town half a day will know who you are and who you're traveling with. They may not like their odds if you get back to the rest of your friends. What's useful? Someone's left their hat behind. Must have fallen off in their hasty departure. Pretty distinctive too. Wide brimmed scarlet hat with a golden feather. Anyone who saw this hat and the person wearing it would remember.

2) Who's really in control here? What here is not what it appears to be? What should I be on the lookout for?

Well, its definitely not you. The lass obviously knows the land hereabouts. She could probably lose you in a second. It doesn't seem to be her either, though. Her eyes are sad as she beckons you onward. She is quite discomfited when you pause to adjust your pack. That's when you notice the change. One moment your looking into those furtive eyes and the next it's as if you're staring into two swirling vortexes that reach into your soul and threaten to unhinge your hold on reality. That's when you notice the knife....