I sometimes taste Wines that I think/consider are over the hill, or on the way down, or should have been consumed earlier.

What are some of the ways, signs, etc... to tell that a particular Wine is just that? What exactly makes that bottle "over the hill"?

Want to see if my impressions are similar to some of yours. And, may be, pick some other clues/ideas/info...Thank You.

...and on this same topic....how can you be 100% certain that the bottle is INDEED on it's way down OR just an "off" bottle (due to poor storage, slightly corked, in a dumb-phase, etc.)?

Exactly & thank you for adding to the question (& stating what I forgot to mention)...

When I find a burnt orange peel flavor in chardonnay, I give it an obituary.

In a red, the color may provide a hint, with brown around the edges. the fruit and tannin will likely have faded and the wines will taste thin.

With red you will see oxidation. The ring around the outiside of the wine in the glass will be the color of brick, orange or brown. It will taste oxidized too like rust; very flat.

The nose on the wine also picks up considerably more savoury characters with reds in particular heading towards a used tea bag type of smell.

Basically as Board-o and Davidn said in my expirience anyway.

The palate moves away form being intergrated to being "thin", ie little body.