Discovering Favorite Treats

Hi All,
First off, I'm sorry if this topic is covered in another thread! I'm new here and I just wasn't seeming to find it! Anyways, I have a GCC name Dilly and Lovebird named Baby that I just adopted. They are both adults, and there is a lot of training that needs to be done with both of them I think. Anyways, I figure that my first step should be finding out what their favorite treats/rewards are. It has been challenging for me to find out what their favorite treats are, but for different reasons.

In the case of Dilly: Dilly won't accept treats from my hand. It results in hissing and aggressive behavior. She also seems to be rather picky in what she likes to eat. Her previous owner said she liked breads/chips, but even if I place something like this in her food dish she won't eat it. She has rejected store bought parrot treats in her dish as well. She'll eat fresh fruits and veggies, but doesn't seem to be crazy about them.

In the case of Baby: Baby has a very different problem than does Dilly. Baby likes to eat EVERYTHING. Whenever I have food out he flies to me and starts trying to devour it, regardless of what it is. He eats absolutely anything that is given to him and doesn't seem to have any favorites. But here's the catch: He also won't accept treats from my hand (unless it is something large like a muffin that he can sit on top of and eat) and will also act aggressively when I try. He'll only eat treats off plates, out of bowls, in his dish etc.

Any advice is greatly appreciated, once again I'm sorry if this a redundant post! Thanks in advance!

Peppers are a great source of vitamins AND they're easy to offer.
Common wisdom is that parrots have such poor taste and smell that hot peppers really excite and stimulate them. And they're very healthy (lots of good green and great nutrents)! So I give the Rickeybird lots! The only downside... when he eats a pepper, then preens, then wants to cudddle, it's a very incendiary experience.
I buy several kinds...
Poblanos, to stuff the end between the bars tightly (above a perch) so the Rbird can nibble at his leisure.
Jalapenos and Serranos, to toss into the cage's snack-bowl, to trick him into entering so I can shut the door. There's no such thing as a chile that isn't good for them. If he absolutely does NOT want to go back into his cage, a big fat Anaheim will always do the trick.
I posted a video of the bird eating one in the thread about beak flavors, and you can hear me sneezing!

Thanks so much! I would've never guessed that they loved peppers! I will try it.