I quite agree that first- and second-growth forests are easy to distinguish in the Eastern mixed forest. Arguably, in my part of the world (northeastern New York), there is no "old growth", since all the ecosystems were severely disrupted in the last ice age. The soil takes longer to rebuild than that.
It's hard to tell old-growth from second-growth in pitch-pine "fire climax" regions. There, the entire forest burns and rebuilds itself every few decades, owing to the inflammability of the pines. Arson is part of their lifestyle. Their cones do not release seeds unless they are burnt: they are the most inflammable of all the trees, and once a generation they burn down the forest to clear away the competition for their offspring.
The subalpine krummholz is nearly all old-growth, since there was never anything economically viable to harvest there. The signs of human visitation persist for centuries, though, because the soil and vegetation are so fragile. But you can often see a fir tree, no more than thirty feet tall and having only a few live branches, that is nevertheless centuries old.
I have the good fortune to have visited patches of woodland left that are fairly undisturbed: never logged, never farmed, not even extensively hunted, by virtue of inaccessibility. There's nothing like hiking in a stand of 300-year-old hemlocks.
I've also had the experience of coming upon a ghost town in the woods that I know nothing about. All that is left are stone foundations, walls, hearths and tumbled chimneys. There were a cluster of houses, some tanning vats, and a mill or two. I've asked some knowledgeable locals, and nobody seems to know anything about who lived there or what they called their settlement. One of these months, I'm going to have to get into the county historical society and see if they know anything. The register of deeds will be hopeless. If you don't know the plat number, you won't be able to find anything, and the property in question has been state forest since the 1880s..
In places like Harriman State Park in New York or Mount Washington State Forest in Massachusetts, you can even find extensive evidence of heavy industry. Those places had been hellholes in the nineteenth century: the land so degraded that it was merely an ocean of mud, all the trees burnt for charcoal or peeled for tanning, iron furnaces belching smoke that hung as a pall over the entire area, abandoned buildings falling to ruin. Now they're quite pleasant again, but the hiker must be careful not to fall into an abandoned mineshaft, implale a body part on a rusted metal implement, or start a rock slide in a defunct quarry. If you know what to look for, there are signs of habitation and industry everywhere!