There are a few different methods you can use to tell. First, the skin of the squid changes when cooked. It won't feel wet and slippery when cooked. Second, when you grab the squid with tongs, it shouldn't drip water when you hold it up. Third, you can test doneness by pricking the squid with a fork and seeing how tender it is. And one last tip: the meat of the squid should turn opaque.