Hi Kirsty,
I always thought it was badgers which were responsible for bringing it in - maybe it's different from area to area.
As for knowing if your hens had caught it - this is what I found -
No clinical signs specifically identify avian tuberculosis in chickens. Infected birds are often emaciated, weak, and lethargic, and they exhibit wasting of the muscles. These signs are similar to those of lead poisoning and other debilitating conditions. Other signs depend on which body system is affected and signs may include diarrhea, lameness, and unthrifty appearance.
Advanced disease and clinical signs are seen most often in adults because of the chronic, insidious nature of the disease.
Maybe other more experienced chicken keepers will have more advice - hope this helps a bit.