Honestly there's no way to really know. You just have to keep working with them.
I have a female non-dwarf burm about two years old now and 8 feet. Her prior owner was afraid of her so my girl didn't get fed much, and when her owner did have to handle her, he didn't want to get bit so he would pick her up by her tail and "bounce" her up and down so she couldn't climb up to his hand. I've had her over a year and she's still not easy to handle and sadly she may never be with her history.
A couple of tricks I did use when she was smaller:
- I would put her in a pillow case or snake bag, lay on my bed, and let her lay on me while I quietly read a book or messed around on my phone. That way she got used to smelling me while in a safe and comfortable place.
- Once she's out I wouldn't put her back until she'd behaved for a few seconds, which usually meant she had stopped striking out at me, shadows, etc. Speaking of shadows, try to set up the lighting in the room with her enclosure so that you're never backlit, as a large dark shape looming nearby = predator.
- If I didn't have time to take her out I would still give her a rub with the hook, empty paper towel roll, my hand, etc. for a few seconds at least twice a day when I checked the snake enclosures to see if they'd made a mess.
- Multiple short sessions where the snake gets returned to its enclosure quickly are better than longer ones.
When my BRB had a litter a few years ago the babies were super nippy; they are known for that. I would open the hatchling tub, make a fist, put my hand down into the tub, and let them wear themselves out on the back of my hand. Eventually the little nippers figured out that biting me wasn't making me go away so they gave up.
Finally I found this video useful:
https://www.youtube.com/watch?v=VDjlg1njtfY