
My dog looked sound the majority of the time when he had a torn ligament. He looked normal when walking, trotting, even running (apart from twice when he held the leg up for a couple of seconds after sprinting after squirrels) but would be lame after resting, really struggling to stand, but then would seem to walk it off very quickly. Other dogs that I've known with the same injury haven't been able to walk, let alone run! The first time we saw the specialist he thought, based on what I had told him, that there was the start of a ligament tear but said my dog looked totally sound when trotting so sent us away for 6 months to try to get the joint to stabalise with scar tissue rather than having to resort to surgery.
The ligament won't show up on an xray but fluid and swelling in the joint would. There is also something called a "drawer test" they do which can confirm ligament damage, but my dog never showed this. I think you would need an MRI to absolutely confirm. The specialist was almost certain my dog had a torn ligament based on symptoms and fluid/swelling on the xray around the ligament site (our normal vets could not see this and sent the xrays off), so he went in for surgery and the diagnosis was confirmed once he'd opened the leg.
Hope that it is not this for your sake (and your dogs!) as it's a long process.