Oh you'll know when theyve eaten something bad! As mentioned above, copious greeny yellow vomit all over the goat and the walls (and you!), frothing at the mouth, general confusion and wobblyness. If its bad they may well go off their feet. You should see these signs within the first 6 hours or so, depending on size of goat and how much theyve eaten. How much food they already had in their stomach is also a factor... the fuller they were with 'good' food the less affect the Rhoddie toxins will have as it will be more diluted.
Make sure you have some activated carbon in stock, and some Milk of magnesia (pharmecy should have some).... if you cant find either.... I cant, seems no shop stocks it up here, the best remedy is strong, sweet cool tea. 7 or 8 bags, plenty of sugar, and luke warm.... let them drink it or if they wont, syringe or bottle feed it into their mouth. the tea has elements which are a strong antidote to the Rhoddie toxins, and you can mix in some plain veg oil and maybe some powedered ginger too. Its an awful looking concoction, but seems to help. The main thing though is the tea - it also helps keep them hydrated whicxh is essential.
Its a nerve wracking thing to go through (just did it - up 2 nights) but as long as they have fresh water at all times (put lots of bowls everywhere, incuding right where they are laid or stood) and you do your best to stop them eating what they vomit again, it should be ok. Trouble is it tastes good to them, so wipe it off their face whenever they are sick.
They will be off their food for a couple of days, but after a day or two should show signs of improvment. As always though, consult your local vet at the first sign of trouble - if they go off their feet a vet will often give a vitamin jab.
If you can get through the first 24hrs, youre probably going to be ok.
Cheers