By now most players know, or should know, that a monster knowledge check can make all the difference when you’re fighting a monster for the very first time. But how can we make them both fun and not break the flow? What techniques can be used to improve the monster check experience?
First of all, it's a Knowledge check
Don't lose sight of the fact that the player asked to make a Knowledge check because he or she wants information. There's no way around that. Don't worry about that information breaking the flow. If the player doesn't want to break the flow by getting that information, he or she would not be requesting the check.
Tease out the information
Take a minute and give the player some information, piece by piece. Make it into a little guessing game where the player is going to get all the information, but tease it out.
"Well, they're humanoid, with orange skin."
"Smaller. And they don't have the pig snouts."
"Some kind of goblin?"
"Yeah, but bigger than goblins. Much bigger."
The goal is to get the player, as his character, involved in the information analysis. Simulate the training the character might have.
Obviously, if you pick some obscure monster that the player has never heard of ("a fell taint? really, that's what it's called? surely that's some kind of joke!"), then you might need to help out more than usual.
Handing them the Monster Manual probably will break the flow, so I wouldn't recommend that.
Make it personal
This is supposed to be knowledge the character has, right? So make it personal. Instead of telling the player, "It's an orc," and giving all the stats the Knowledge check provides, add in some history or back-story.
This is a golden opportunity for a Dungeon Master. Don't squander it!
"I make a Knowledge check. 29! What is it?" "Well, your mentor, Grim Wizard Horace, used to talk about these things all the time. He said they were wisps of insubstantial evil that ripped through planar material." "Wow, they sound dangerous." "Sorta. Probably manageable. Horace said they weren't that tough for him and his adventuring party, the Wrecking Crew, and you're as powerful now as he was then, probably. But he warned of one thing." "Oh?" "Yeah, he said that when their paladin, Golden Boy Gabriel, fell unconscious, the damned things became material again and started feeding on poor Gabe. Sucked his life out of him." "What are these things called?" "Oh, you'd never forget that. Horace called them 'fell taints,' though you're not sure if those are the official taxonomic designation or if he was just being crude."
Often I have a bunch of PCs all want to make the check, to get as much intel as possible. So I do the checks in secret and write them notes, with significant failures being rewarded with misinformation, and then am entertained by them trying to divine who is right. "The wizard's usually pretty smart but 'fell taint'? Really? Both Bob and Dave the fighters think it's a dread wraith, let's go with that."
(Amplifying my comment)
By handing players aspects of narrative control of the monsters, it can be possible to increase player investment in the setting. While the most trivial check should result in a small card being handed over for the monster defenses (because that just speeds up combat), more significant checks should allow players to describe the monsters' attacks.
Interesting monsters almost always have more than a simple at-will that they spam. While some of the encounter powers may be relevant to the story, others are merely "hey, that's cool." By letting players take ownership of the "hey that's cool" and, more to the point, describe the source of their information, they'll then start fleshing out parts of your world or their backstory for you.
The higher the role, the more generalized the addition. A "moderate" roll might be enough to describe an encounter power. A "hard" roll might go to differentiate a sub-species of monsters. Don't worry about balance as the players are not describing the mechanistic effects (use the Dm's cheat sheet for that)
For more details about this sort of play, check out the indie game Donjon, because it has quite a few aspects that could be profitably imported to 4e, and knowledge checks are the right way to hook those features in. It's also a way to make "known" monsters new and interesting for both the players and the GM.
In other systems, making a knowledge check lets you define a characteristic about the monster. I haven't actually tried this yet (mainly because I don't trust the wizard not to assert that every monster the group fights has lightning vulnerability), but it's certainly an interesting way to handle the problem.
I would give a successful knowledge check a heads up before the creature makes its next attack or give some kind of warning about what it is going to do next. "As you are dodging out of the way, you notice the scales around the throat flare, it is getting ready to breath fire." "An old memory from a bar long ago, the description of how a beholders eyestalk stiffens before it attacks - JUST LIKE IT IS NOW!!"
Or I would use a successful knowledge check as a negative modifier for the monster, as it is more predictable or could be anticipated. Depends on the situation.
You could allow players to use their knowledge modifiers as a bonus to combat rolls... kind of like the 3.5e Ranger's Favored class.
So if my character has +2 Knowledge(Beholders) - or whatever the actual knowledge skill would be - I can get a +2 to Bluff, Listen, Sense Motive, Spot, and Survival checks as well as damage rolls.
So at the beginning of combat just declare "if you have Knowledge(X) treat it as a combat bonus."
Now the characters with the appropriate knowledge can treat it as an implicit benefit, and the GM can then provide information as narrative; the character with +2 Knowledge(Smaug) rolled a crit? Narrate that they strike the left breast - knowing that's where the dragon's armor is weakest.
If PC's have no way knowing, simply don't allow them a check outright, but "advice" them they should observe the creature a little longer. In stead of a lot of attack rolls, you're going to make a lot of (hide/move silent/spot/knowledge) skill checks first, where successful knowledge checks actually represent correct interpretation of its' behavior, in stead of summing up an MM-entry. Having 5 ranks in Handle Animal or the like, could result in a synergy-bonus for knowledge (whatever relates to creature).
This does however, adds that little extra nasty flavor to PC's being surprised by the monster.