Some changeling powers are decidedly un-subtle: bursting into flame, floating to ground like a leaf, making trees bear fruit in winter, etc. Others are much less obvious in their use, but still might draw attention, like boosted dice pools or lightning-fast crafting. There are no hard-and-fast rules for adjudicating who notices what, but the guidelines here might help.
When a changeling's magic is noticed, they trip the Clarity 6 violation, "Obvious displays of magic in front of witnesses". Changelings below Clarity 6 don't have to worry about that, but they can still get in trouble with the local mundane and changeling authorities, or have to do some difficult explaining to their human witnesses.
Although most contracts and pledges have no blatantly obvious effects, they introduce advantages that can set the changeling appart from everyone else. Keep in mind that the raw bonus to a dice pool is entirely opaque: it's the rolled successes that are seen.
First up are the n-again rules: 10-again, 9-again, and 8-again. Everyone in the world naturally has 10-again, so that's never an issue. 9-again starts to get fishy, but will typically be rationalized as mad skillz or great luck. Be careful about directly contradicting someone's expectations, though.
For example, a lousy poker player starts using Hearth contracts to gain 9-again for the most important games. Her reputation belies the incredible skill and luck she displays at these games, which draws attention. Egregious abuse might even cost her Clarity.
That brings us to 8-again. This benefit is dangerous for an undercover changeling, because 8-again almost always gets noticed. The only mundane activity to benefit from this rule is a Firearms roll using one of the best sniper rifles in the world. Hence, gaining 8-again on pretty much anything will shock nearby humans, earning their attention and risking your clarity. Getting fewer successes is hard when you have 8-again, but see Concealing Magical Help, below, for suggestions.
Extreme levels of skill are much easier to excuse for a long-running task than they are for a quick action. Unless you draw attention to how little time it took you to rebuild your car, for example, no one will guess that you had magical help. Even if people do notice how quickly you accomplished an extended task, most suspicions are easily brushed aside by claiming, "It wasn't as bad as it looked", or, "The shaft fit perfectly on the first try! That never happens!". Just be careful not to try that too often.
On the flip side, the Curse line of pledge sanctions can also be a risk. Losing 10-again isn't so bad, and neither is only succeeding on a 9 or 10: everyone's had a week like that. Just be judicious, and remember that it can arouse suspicion in the wrong circumstance.
Only succeeding on a 10 is the worst luck imaginable and goes well beyond mere bad luck. Whoever you cursed will know something's up (if they didn't already), as will every human who interacts with them. There's simply no way to suddenly suck so badly at so many things in life. There's no immediate risk to your clarity, but expect some well-deserved attention if their luck can be traced back to you. If you're obviously connected to the victim, this attention will come sooner and possibly ding your clarity.
For a quick explanation of the risk of a big dice pool, let's take the easiest example to codify: Firearms. The best human marksman in the world, when using the most powerful (personal) gun in the world, can count on having 19 dice (with 8-again) for his Firearms attack roll: Dex 5 + Firearms 5 + Weapon Damge 6 (8-again) + Specialization (1) + Sighted In (1) + Good Range (1). And that's before actions like Take Aim. That's damn high. For comparison, a competent changeling can get to 18 dice (with 9-again) with some training, a normal gun, and a pledge: Dex 3 + Firearms 3 + Weapon Damage 4 (9-again) + Adroitness 5 + Sighted In (1) + Good Range (1).
The difference is that the human in this example is the best in the world, with tens of thousands of hours at the range and equipment priced to match, and the changeling is some schmuck with a hunting rifle from BJ's. A similar example can be made for other skills, where the best in the world will have 10 or 11 dice, before equipment, and the average changeling can easily match (and exceed) that pool through pledges.
The human in this example also has 8-again, while the changeling only has 9-again. This can make a big difference in rolled successes, but it doesn't change the fact that the otherwise mediocre changeling can compete with the world-class human.
A huge dice pool isn't as noticeable as 8-again, but a mortal who's well-versed in the relevant skill will probably notice your exceptional prowess, and anyone who knows you well enough will probably ask how you got so good. They may start wondering why no one has heard of you, or why you're on the run, and who did you used to be? It won't cost you clarity unless you do something stupid, but it'll complicate mortal relationships and may attract the wrong sort of attention.
To mitigate huge dice pools, you can always choose to intentionally do worse. In game terms, you can subtract one die from a dice pool for each real dot you have in the rolled skill. In some circumstances, that might bring you back down to what humans can do, or at least help convince people that what they saw was just a fluke. If a bystander is already suspicious for some reason, or if they're just paranoid like that, you'll have to make a Subterfuge roll to pass off your lower dice pool as "just the way it is". In particularly bad cases, this might be opposed by the bystander. If you fail, the jig is up and they know you're hiding how good you are.
Another approach is to dress up your prowess with a good story, or supporting materials. Sound like Madonna? Hand out some CDs and say you're hiding from the paparazzi, or are from another country. If you can explain why they haven't heard of you, you're probably set. Just be careful of the mortal rumor mill: there are few things more juicy than a secret superstar.