What does the domestic price that prevails without international trade tell us about a nation's comperative ad?

Your wording of the question seems to be wrong. However, if you know what you will pay for a domestically priced good and the same good produced overseas, you will see where the comparative advantage lies. If it costs you less to buy the domestic good. Your country has the comparative advantage in producing that good, all other things remaining equal. If it costs you more then the overseas country has the comparative advantage, all other things remaining equal. Now, if you don't have international trade. You won't know whether you have a comparative advantage or not with a domestic good because there's nothing to compare it to.