Cash knocked her with his shoulder (not hard enough to knock
her down and didnt look like he intended to do so)

In my experience, dogs are very body-space aware. They may choose to
ignore your or another dog's personal space, but they know when they
are doing it. The hip-check and shoulder-check may look accidental,
but is all part of how the dog is taking and using that space, and at
times, controlling the body position of the other dog/human, or at
least asserting their right to do so. It can be playful, it can be
assertive, it can be aggressive, but it is rarely accidental.

Other things to watch for are the head fake, the air-nip, air-bite,
all signs that the dog is simply saying "I'm not gonna...this time,
but, you know, I could...if you push me". Again, it can be as much in
play as anything else. Then again, establishing pack order is part of
the play for them "hey, I'm having fun AND I know where I am in the
pack" (hip check, play bow, bark) "...chase me?"

It's the ones who go all the way to sinking in the teeth without
letting you know ahead of time that "hey, you're pushing my limits,
and I think you should know" that are especially worrying. Sometimes,
though, it's just a case of us dumb humans not paying close enough
attention and not knowing how to read the signs.

I notice it with my girl dog much more than with my older male, there
is a constant test to see if you're paying attention. I'm not sure if
that's a young dog thing, a girl-dog thing, or a lab thing, or just a
dog thing in general (I suspect, the rest probably just determines the
degree).
--Glenn Lyford