Head gasket is usually shown in various ways from loss of coolant for no apparent reason - milky residue in your oil - oil in your expansion tank - running on 2 or 3 cylinders etc. Can be just one or a mix of all of them.
Just for you guys who aren't too sure, it's no big deal that a head gasket goes, and don't worry about it causing more problems later providing you get a pucker job.
The main thing people scrimp on is having the face of the head checked to see if it's still 'true' and then decked/skimmed where necessary. If you don't due to the head being alloy and the block Iron, it's possible for the head to warp and effectively twist when torqued back up. This will be a tiny ammount, but if unchecked when you bolt her back up you get problems with over heating etc and more importantly you run the risk of the cam seizing where it's bent!
Do get your head skimmed will cost about ?25, so if you're planning on keeping the car then insist it's done, or atleast check!