Let's get the correct translations in here:
hapter: 486 (DBZ 292), P2.2-6
Context: after Boo transforms into evil Boo
Piccolo: “…Have you noticed? …This change in Majin Boo’s ki…[ ] …He’s changed…All due to some idiotic Earthlings…He’s become pure evil, and his body has become more suited toward battle…Th-this…this…”
Kuririn: “Wha…What?...D-don’t tell me you’re saying this is bad!? …It’s al-alright! We’ve got the squirts’ Fusion! Goku said that right? That Fusion was the strongest…!”
Dende: “…”
Piccolo: “…That’d be nice, but…”
Piccolo already is doubting Gotenks here. Krillin clearly doesn't understand what has gone on, but Piccolo has already realized that Gotenks doesn't have a shot with fusion.
Chapter: 486 (DBZ 292), P3.1
Goku: “…This is Majin Boo, right? This ki…what is it?...”
Note: mostly I wanted to point out that Goku and co. can sense evil Boo from the Kaioshin Realm.
Boo is felt from the Kaioshin world when he flares his aura.
Chapter: 487 (DBZ 293), P13.5
Context: as Piccolo plans on having Goten and Trunks train in the Room of Spirit and Time
Piccolo: “Can’t you tell…?! As they are now, even if they perform Fusion they can’t win, no doubt about it…! Tell them that if they don’t want to die while they’re still just little brats, then they should train as much as they can…!”
This goes along with the earlier point. Piccolo knew from the start that Gotenks was no match, whereas Krillin and the like still can't figure it out. Boo doesn't power up until he enters the RoSaT, as Gotenks was confident that he could match Boo in Base, and Piccolo was even shocked by how far they had come.