Head Gasket?

Hi I have a 300tdi Disco with 112,000 miles on clock. A friend of fine says he has spotted a bit of white smoke coming from exhaust when vehicle is climbing. I regularly check the water and of course keep an eye on the temperature guage and no worries there.
Could this be a sign of head gasket failure?

white smoke from water is noticeable in that it disappears straight away ,white smoke from unburnt fuel hangs about check which

Might just be some damp fuel. Like James says, does it stay and linger?

thanks i will keep an eye on it