if its a dealer selling it then i believe they are obliged to tell you, but as a cat d write off in the past with a full v5 it should state repaired vehicle or similar on the document. if no v5 be careful as it has to have a vic check but has to be repaired to a top grade to pass this test.
on the plus side though if the v5 is there and all checks have been done then you'll be able to purchase it at a lower cost than normal as people tend to stay away from write off's.