Generally Valve Seals last roughly 10 years or about 150,000 miles. If you don't use the car they will dry up eventually. Usually you don't know the Valve guides are bad until you go and take the valve seal out and the Valve Stem comes out with it. From my experience it is usually the rear stems that come loose first, due to more heat staying in the back of the head. The Valve Stems are made of Brass and the Head is made out of Aluminum, so after years of use the Valve Stem may come loose due to the Brass and Aluminum having two different heat expansion rates and cooling rates. You will get oil on the plugs if the Valve seals are bad and you will get oil on the plugs if the valve stems are really bad. If the guides are really, really worn out they can come loose around the valve and the engine will make a ton of noise then, and you will more then likely need a new head then, but this is really rare. The Valves almost never need to be replaced unless the engine is over reved and the get bent. There is no need to pull the head off just to check the Valves. If you need guides, then you have to pull the head off and the head will have to be sent to a machine shop to have new guides pressed in. In most cases you will just burn a little oil, if it gets really bad you will start to see some blue smoke coming out of the tail pipe.

These are the most general things, I am sure someone will come up with something I am forgetting at the moment!