If it runs fine and has a "sort of" white smoke, it could be just condensation...
If it runs bad, it's a head gasket, most likely... Did you overheat it lately???
Just to let you know, take off the gas cap and take a sniff, then sniff your exhaust pipe... If they don't smell the same, then it isn't gas coming out of your tailpipe... All kidding aside, it would be evaporating before it would make it to the tailpipe exit or be a giant flamethrower, LOL...
Mike