It looks like US reserve growth is declining even when accounting for all the offshore stuff discovered in the last 30 years. So we're not finding as much as we used to.
Edit: Sorry, I misread your question at first. "Peak oil" refers to peak production. We know we passed this in the continental US in 1971. That's only knowable after the fact, of course.
From what I've read about the peak oil phenomenon, it seems like sort of a coincidence that peak production happens around the same time half the oil in an oil well (or oil field, or oil producing region) is exhausted. So that's how people try to predict future peak oil events.
The problem is that proven reserves are a moving target. (Though we know the growth of reserves is trending down for the US.) And it's harder still to know much about the rest of the world's reserves since oil reserves are regarded by many nations as state secrets.
