You might able to figure out the horses on the extreme ends of the scale, but for the average-type a-bit-hot/a-bit-dull horses I don’t think you can tell when they’re young.
I’ve had pretty flamboyant young horses end up being dull and I’ve had pretty unperturbable horses wind up with an engine.
If I had been told to guess about my 4yo when he walked off the plane/trailer, I would have guessed that he would be a bit dull because despite never having been off of his home farm as a youngster, he was so chill about the import process and took all the new strange things in stride without acting terribly reactive to anything. But under saddle he’s got a big engine and wants to go. With that being said, I think once he’s been under saddle a bit longer he’ll be one of those that sits right in the middle - don’t have to push him to go, don’t have to whoa him to slow. And as I worked with him over the first few months I think I could get to that if I were guessing. Though he’s not spooky or overly reactive at all, he’s also not dull to work with on the ground. But that’s figuring out what a temperament will be AT the age that you ride them, which is a different story.
I got my (now 6-year-old) mare as a coming 2yo and I was positive that she would be a fiery one. I started lunging her lightly as a late 2yo and she was wickedly fast with her hind end and put on some bucking displays that left me a bit worried about the breaking process. She’s still reflexively fast (she SNAPS her legs up over jumps) but she’s a push ride all the way. She has also never bucked or done anything naughty under saddle, and I was POSITIVE she would be a wicked one.
I had another horse I bought as a yearling who I thought would sit in the middle. He was spooky and reactive as a young horse, and wasn’t particularly hard to make move. But he was shockingly dull from the moment I started working with him. And I’m pretty good about motivating dull horses, but even with me riding him every day from the beginning, he still required a lot of work every day to keep moving appropriately forward. Now, after he got through his first couple of years under saddle the spooky reactive side of him showed back up and I ultimately had to sell him as a hunter because hunter fences/rings weren’t spooky to him while jumper fences were. But nothing in his early years indicated that he would be so dull.
Perhaps some are a bit obvious as you work with them by how hard you have to work to get them to move. And I’ve had a few where their performance on a lunge line was quite indicative of how they wound up going under saddle.
But I guess I view it the same way I view free jumping. The work you do on the ground MIGHT be indicative of how they’ll work under saddle…and it might not.
I agree that starting with the bloodlines is a good place, but that’s not always a match either. I can say that my breeders focus on breeding a “trainable” mind, and WOW have my recent young horses been easy to work with. But that factor has little relation to the amount of go the individual horse has. I guess it does mean that none have been on the extreme end of the scale, but it’s a fine line between “a bit quick” and “a bit slow.”
YMMV, of course. Especially for those that do a lot of ground work before breaking a horse. I don’t do a lot of ground work with mine beyond a few sessions early on and general farrier/vet sessions.
Oh and I have a theory (that I’m sure a lot of people can bust into a million pieces). I think that truly intelligent horses generally (not always!) end up tilting more toward the dull side. And that’s because I think really smart horses understand that they don’t need to be reactive or, more importantly, over-reactive to stimuli.
But that’s a whole 'nother ball of wax. Because some horses are “clever,” some are “smart,” etc. And I don’t think I can consistently differentiate between the different types of intelligence at an early age.
So that was a really long, rambling way to say, “I’m not sure.” 