How did Elfangor know that Jake and the gang were not Controllers?

[Phoenix004]

Their reaction to an Andalite landing in front of them kind of proved they weren't Controllers, but also the fact that he saw 5 lines join his brother's when the Ellimist moved him through time/space. Obviously these 5 humans were those lines, so he knew he could trust them.

Besides that, it was simply a calculated risk. If he did nothing, Earth would've been screwed, so what did he have to lose by giving them the morphing technology?

[warren_bearclaw]

so what did he have to lose by giving them the morphing technology?

the war. And yeah, it would have been a really short series (or single book ) if he didn't.
Another point to bring up: Remember that when they met Elfangor they all were just filled with trust, but with Visser Three they could just feel he was evil. Maybe he could kinda do the same thing. It doesn't really make sense, but then again... Also, Tobias had that special connection with him. Maybe Elfangor felt something too.

[Hawkgirl]

hmmm... yeah, but still, he only saw that Tobias and the gang would intersect timelines, he didn't see what would happen. If he did, then he would have known that he would die at the construction site and give them morphing power in the first place.

Didn't Elfangor know he was dying at the consturction site?  He could sense his life was going to end and suddenly, these five humans show up.  I would think that if Elfangor knew he was dying and 5 human kids show up, he would know that these were the 5 timelines intersecting that the Ellimist showed him.  Especially if he felt the connection with Tobias that Tobias felt with him.

[jiminy]

Back in the day, when the official website had the "Ask K.A. Applegate" feature, I actually asked this question and they chose it for her to answer!!  Her answer was basically exactly what you guys are saying:  that a real Controller would not have been able to hide his disgust and tried to Dracon Elfangor.  She did admit though that what I said was a very good point, which getting a compliment from her made my 10-year-old self's day, haha.