It can be hard to tell because of their different personalities.
Like, I think both Michi and Pasha are bonded to me, but they show it in different ways. Michi is so brave and outgoing. When she hears me near the cage--even during the middle of the day--she pops out of the sleeping pouch to see me.
She prefers to eat her treats while she's sitting on me and I'm petting her the whole time.
In the tent, she will have little bursts of energy and want to explore, but she always quickly comes back to me. She probably spends 75% of her tent time sitting with me, grooming my hands/fingers/toes.
Pasha is a little shyer. She won't come out of the pouch in the daytime (though she'll crawl up to the very edge). I almost think it's something to do with her ruby eyes being more sensitive to light, because the flash of the camera bothers her way more than it does Michi.
Pasha will explore more on the tent, and use me more for a launching pad, but she will always run back to me when startled. She rarely grooms me.
Now, you can see that if I judged Pasha by Michi, I'd think something was "wrong" with her. But that couldn't be further from the truth! Despite being twin sisters, they are just very different little souls! They were even at the breeder's; Pasha was a crabber and Michi has never crabbed a day in her life.
Ultimately, it sounds cheesy, but I want to just say that "you'll know." As I got more comfortable with them (they were my first gliders) and they got more comfortable with me, it was like one day I realized how much I trusted them and how much they trusted me. It was a nice feeling.