I'm not going to say this is a definitive list of symptoms but its what I observed on my car,
1. Little by little the engine seemed noisier, not a lot, when the lifter started obviously leaking oil through there was slightly less pressure held in the adjuster so the noise got louder, the tolerances get further out and the failure starts to accelerate.
2. For me also the black soot that used to accumulate around the exhaust started to get a little gummy so I guess this was diesel and carbon mix due to incomplete combustion because the exhaust valve wasn't lifting like the others.
3. Eventually when the adjuster went through and could hold no pressure the car felt like it ran on three cylinders as the exhaust valve would be barely opening. There was also a loud thump from the inlet manifold intake - I guess this was exhausted gases that weren't exhausted exiting via the inlet manifold instead when the inlet valve opened.
My 2cents