Realistically I don't think there's a lot that can be done.
If you're really worried then you can ask the vet to do a blood test for liver function so you know what you're dealing with, but as far as I know that won't differentiate between new & old damage so would be no use to you if you wanted to take action against the supplier. If results did show damage you could alter the diet to minimise challenges to the liver & maybe supplement to support it, but liver damage from ragwort is irreversible because chemicals in the ragwort also stop the liver from regenerating.
First of all check that it's definitely ragwort, hopefully it's something that looks similar when wilted & you're worrying for nothing.
Re the supplier, I don't know. Any field could have a patch of ragwort in it even if the farmer is careful & if it wasn't in flower & was surrounded by long grass it would be difficult to see. If you haven't had a problem before then changing could put you in a worse position. If there was loads in the bale I might feel differently though & at the least I'd be asking for all the bales to be replaced with bales from a different field. Being a cynical soul I'd also mark the bales with a permanent marker to make sure the same ones weren't returned.
