Easiest way is to note the players perception skill (and other useful info) on a stat reference block and make the roll yourself.
This means you're making a roll for some reason however, which may get the players meta hackles twitching.
An option to avoid this that doesn't even involve rolling a dice (if you don't even want that to be seen) is pre-roll a load of d20's on a scrap of paper; cross each one off as you use it in turn for these "secret rolls".
Or you roll extra d20 random rolls all the time, for no reason at all. This has the added advantage of making the players more paranoid ;) (this is rather than the players making extra pointless rolls) you can speed up this with coloured dice with one colour per player and roll a bunch of them.
Hopefully however players can simply ignore these perception checks without the meta.
Related: ( How do I use Passive Perception to have some characters notice parts of the environment? )
Never forget "Never ask your players for a skill roll you don't want them to fail."
There are two ways to interpret that sentence, and it hinges on a weirdness of how English uses the indefinite article.
Any character or monster that doesn’t notice a threat is surprised at the start of the counter.
The usual interpretation of "a threat" here is that it means "one threat". If that is the correct reading, your question is the result. Is it correct though? This meaning would require that surprise is a relationship between two individuals, so that the dire wolf could be surprised by the rogue but also not surprised by the enchanter.
Is this how surprise works? It turns out, no:
If you're surprised, you can't move or take an action on your first turn of the combat, and you can't take a reaction until that turn ends. (PBRv0.2, p. 69)
Surprise is not a relationship between two entities, it is a state of a single entity. It's impossible to be surprised by one opponent but not surprised by another.
Is there another way to read that sentence about "a threat" that makes surprise sensible as a state? As it turns out, yes.
"English is funny that way"
Another use for the indefinite article, which looks identical to the "one threat" meaning, makes the sentence in question
Any character or monster that doesn’t notice a threat is surprised at the start of the counter.
mean any threat at all, not just one. If this meaning of "a threat" is how it's being used, then that means only creatures who notice no threat are surprised.
Because this reading is perfectly normal English, but ambiguous, we need to confirm the reason by looking for clues in the surrounding text. That confirmation is in the definition of surprise we looked at above: being surprised means being completely surprised, which only makes sense if it happens when no threat is noticed.
So the dire wolf is not surprised, because it did notice a threat, as opposed to not noticing a threat. (See how that makes sense put that way?)
On the plus side, the champion and the rogue don't need surprise to have advantage on the dire wolf, because that doesn't rely on surprise, but rather on being unseen, and being unseen is a directional relationship, not a state.
Best Answer
In D&D it has no name, because it's not part of the game’s rules — you made it up, much like many other GMs before you have made it up.
There are other games that include this mechanic though; some games call it a “graduated” roll, others call it an “open” roll, and others yet might call it something else. Some games feature degrees of success as a natural part of their system (and so don't bother to specially-name this normal kind of roll), but you could synthesise the term “degree of success roll” from that precedent. There’s no standard, accepted name for this mechanic. I've heard it called an “open-ended roll” too, though that’s ambiguous and can also refer to an exploding dice mechanic. In some roleplaying groups this is informally called “rolling for how well you did”.
So since it’s a house rule specific to your table, and you can call it anything you want! I like “graduated” or “graded” roll myself, but your taste is what matters.