@sourland made a
good suggestion to
watch closely. It also depends on the dog, how long have you had chickens (and dog), and
@Hei 20 posed some good questions as well..since we don't know the exact details.
For some breeds, even the most well-behaved dog has instincts and most times accidents happen when you're not around to immediately correct them.
I am not a professional dog trainer but since we got chickens (4 months ago) we have been working with our dogs as much as possible on simple commands like, "leave it" or "ignore" or, mostly the Cesar Millan "ttssss!"

when our dogs got ahold of our pullet, it was a mere response to the sudden movements when she squeezed through the gate (initially thought she flew over the gate) and tried to flutter back. I was heartbroken but it wasn't the dog's fault. I should have enclosed the chickens better but I didn't plan as well as I should have.
Another user mentioned positive training, instead of just commands alone, to help encourage the desired behavior; so, when I'm outside and the dogs are ignoring the chickens, I give them a treat! Even if my dogs were completely trained, I don't think I would ever 100% trust them to be full-proof
alone and unattended with the chickens, especially knowing the breed.
It takes time and
consistency is key!