How did Boba Fett knew to wait in the debris?

He wasn't waiting. He just likes hanging out around in garbage and just happened to get lucky. He's like a cosmic Fred Sanford.

I suspect, Slave 1 was out in the distance from the Imperial-class Star Destroyer and was able to see Han Solo's entire maneuver of kamikaze straight for the ISD's bridge, and then swiftly maneuvering to grapple the Falcon on the backside of the bridge tower.

Boba then simply waited for Han to make his move, and then followed him all the way to Bespin.

I'm guessing that they had originally planned on making at least 1 sequel to Solo, but when that was a disaster all future movies were scraped. Anyways, I had hoped there would be a scene in Jabba's palace where young Han will be regaling Jabba with a story of how he escaped capture by a Star Destroyer and completed his mission by doing that exact maneuver. And he knew from his days in the Imperial Navy that they would dump their garbage before going to hyper drive. Jabba of course loves the story since in resulted in him getting paid. As Han leaves Jabba's throne room, he walks past Boba Fett who was listening very intently to the story.

Fast forward to ESB, after word gets to the bounty hunters that the Star Destroyer Avenger was pursuing and lost the Falcon, Boba would request the ships last known trajectory. Knowing that they probably couldn't go into light speed, because they would have done so much sooner, but some how just dropped off of radar. Boba flashes back to Han telling Jabba this story, and has a good beat on where the Falcon is. So he hid the Slave 1 in the garbage from the other Star Destroyers and waited for them to leave and the Falcon to make its move.

The Falcon flies up real close to the star destroyers before "disappearing". If you're close (in formation in the Imperial fleet) the Falcon zips up, zips past, and "poof!": gone. But if you're further out, you watch it loop around and attach to the back of a star destroyer. My guess is that Fett was lurking around outside of the fleet's perimeter and just literally watched Solo do that. He was waiting for the right time to strike because he couldn't fly over without alerting the Falcon (they'd see him and rabbit, fast), so he was waiting for Han to make a move. Watching the Falcon sticking to the back of the star destroyer, Fett could figure out what Solo was up to and was just waiting at that point. Why not alert the Imperials? Fett wanted the best deal possible, so he alone would get the credit and the collar. While tailing the Falcon to Bespin, he probably sends a message to Vader and says, "Meet me here, talk to Lando, you'll get Solo guaranteed," and he cemented his reward.

Because his mom said "Have you looked in the last place you saw it?"

In-universe, a young Boba Fett saw Obi-Wan pull the same move just before the Clone Wars started.

Really?

I was 10 years old when I saw this which would make me an unworldly kid at the time and even I knew immediately that a character like Boba Fett probably had the same mental acuity and galaxy-smarts as Han Solo. What little I knew about the concept of smuggling commerce across controlled territories led me to know that both Boba Fett and Han had similar mental constitutions grounded in underworld experience and knowledge. I didn't speculate for one second WHY Boba Fett would be wise enough to figure Han Solo using the hitch trick on the Imperial Cruiser fleets.

The plot required it.