Tattle

If you've played Paper Mario, you know how this works, but if not, the first partner that Mario would meet in his adventure would typically have a special attack called Tattle, which was unique among special attacks in costing 0 FP. This is because it doesn't deal any damage, but instead provides information on the targeted enemy, with the additional feature of from then on making that type of enemy's HP bar visible during battle. Thus, in order to be sure about how much damage you've dealt and how much more it will take to finish enemies off, each enemy species must be Tattled on at least once in a playthrough, although there were some ways of circumventing this. From the first game to the second, this became a little more of a collectathon, with a "Tattle Log" provided in the pause menu showing all the enemies you've Tattled so far. Paper Mario is not the only RPG to do something like this, although many still refuse to reveal enemy HP, making fighting them more of a guessing game unless you've played through the game a bunch of times.

A game of Arpeggio may be run in the latter fashion with enemy HP never being visible to the players, or it may be visible from the start with no Tattling necessary. But for a true Paper Mario feel, one party member should have Tattle as one of their special attacks, or a differently-named attack that functions similarly. If players are reluctant to spend AP on it, an alternative is to create some kind of item or weapon that allows them to achieve the same effect; I've got one called a Bestiary, based on the book seen in Goombella's Tattle animations from PM2. If Tattle information is coming from a book, it would be possible for some enemies or bosses to not have their stats included in the book (in theory due to its author not having encountered them), making Tattle unusable on them. In this scenario, there may exist multiple Bestiary-type items, each of which contains information on a different set of enemies, which would leave a foe whose data can't be found in any of them with a real sense of mystique. On the other hand, the book could be presented as a set of instructions for assessing enemy stats, getting around the question of just who is this author who's evidently encountered all this stuff before but neglected to accomplish any of the players' goals ahead of them.

Bug Fables offers another option: any party member can use Tattle (there called Spy) for free, with each having something different to say about each enemy. This would not require anyone to spend AP, but rather would be a separate action that can be taken in battle, like Running Away or Doing Nothing. Normally, in Arpeggio, only one party member would need to Tattle/Spy on an enemy in order for the entire party to then see its stats, but if you want more of a challenge, you could separate it such that only those player characters who have Tattled/Spied can see the stats. This is mostly pointless since those players could then just tell the stats to any remaining players, but in Game 1, there was an interesting scenario where one of the player characters had a high Knowledge stat, which I as the Maestro used to say that he should already know the stats of any items that the group encountered, but the character, though sentient, was mute, meaning that in order to convey known information to his teammates, he had to essentially play charades. You can see how this could also work with enemy stat information, so keep such possibilities in mind, whether you're a Maestro or a player.

The default attack write-up for Tattle is as follows:

• Tattle~ VP: 0, Target: One, Reveals enemy stats, weaknesses, and trivia
  "Uses visual clues to discover an enemy's threat level, weaknesses, and personal information."

While the standard version is costless, I would charge 3 VP for a multitarget version that Tattles all enemies in one turn. You can, of course, create a magic version that costs MP instead, or a psychic version. Paper Mario's precursor, Super Mario RPG, had a similar move called Psychopath, usable by Mallow; this only momentarily revealed an enemy's current amount of HP, and revealed a quote from the enemy's thoughts rather than any particularly useful information about it, but it could probably serve as a psychic version of Tattle in Arpeggio.