How can I tell the difference between a puppy being aggressive and a puppy that's just really annoyed, if that makes senses?

We adopted Lila from animal control at 6 wks (she's now 8wks), and I knew that the strength of her bite would probably be something fierce since she didn't get a chance to learn all the puppy-know-how she would get from her litter mates. Lila is definitely of unknown origin, they had her listed as a lab/shepard mix, which she is obviously not... but we believe either lab/pit or boxer/pit. But I have discovered that she is very headstrong and stubborn and MOUTHY , but very food motivated...she's already caught on to "sit" and is starting to understand "down".

Anyway, I picked up a head collar for her because she has been rather stubborn with a regular collar. After all the choking she'd been doing, it was recommended by our vet. She's obviously not happy with it and will rub in the grass, scratch at it, etc. Today, she managed to get the strap that goes over her muzzle off (it obviously wasn't fitted right at the pet store). When I went to put it back on, she went crazy...there was no growling but she was trying to bite the collar, me, etc...and she would not back down. I know she was worked up, but how do you tell the difference when a dog is rebelling compared to a dog being truly aggressive. I am very vested in Lila, I wouldn't have adopted a dog if I weren't...but I do have 2 children (7&1) and can not take the risk of an aggressive dog in the house.

And how do I control her attitude with out "breaking" her or making our relationship confrontational? I've had dogs before, but none have had the headstrong attitude that she has and I am at a loss.

Thanks in advance