How do you know if you have a blown head gasket, and what do you look for. 91 honda civic, oil on 3 spark plugs, not the 4th. And runs but runs rough.

Remove spark plugs, attach a compression gauge, crank engine, and record each cylinder's compression. Oil or antifreeze being burned are signs. So is antifreeze in the oil pan. If you know the engine really overheated, you can likely count on the head being warped and the gasket being blown.