Hmmmhh.... "they are used to being tethered..."
Ask about - breed, vaccination regime, worming (and if necessary fluke) programme, see if their feet are overgrown - if answers are evasive - walk away.
check them over - are they thin (can you see their ribs and do the hips stick out) - if yes (I take it the girl is not being milked atm), again walk away. (a dry goat at this time of year should have a good covering on her, without being fat). A thin goat (esp a wether) could mean not wormed recently for example. See what their droppings are like too - runny, again walk away. Pick up their feet and check them for scabby bits just above the hoofs, foot rot and how recently trimmed.
If you want to breed from the female - check her teats, they should only be two and each should only have one orifice. No fishtail/double teats - impossible to milk. (If the wether is her brother check his teats too)
Horns?
If sold as registered - ask to see the female's registration certificate and take down her number, so you can double check that goat is with the right owner. (I can check for you)
Really the best would be to take another experienced goatkeeper with you for advice if you are new to goats.
See what their housing is like, do they share with sheep atm...