If you know at least one non-obvious attraction that you are interested in, you can use this to “calibrate” online or offline guides for the area: if a guidebook doesn't list X, it means it fails to describe the properly off-the-beaten-track areas.
A guidebook on the Grand Canyon that describes the North Rim as off-the-beaten-track, is not a guidebook I would buy. Although I readily believe it's quiet compared to the South Rim or Zion, it's still quite crowded by my standards. But a guidebook that spends at least several pages on the Tuweap area is something I would consider buying, because chances are a guidebook that has details on areas like Tuweap (that I do consider off-the-beaten-track, at least compared to the main destinations on the Grand Canyon; we hiked all day with canyon and river views, met zero other hikers), is also likely to have information on other off-the-beaten-track areas that I don't yet know about. Alas, the guidebooks for sale in the visitor centre only mentioned Tuweap in passing, so I didn't buy them.
Similarly, a guidebook for Iceland that has information on Lónsoræfi or Þjórsárver, is one that I would buy. Yet if it's limited to "off the beaten track" (not really) destinations like Þórsmörk or Snæfellsjökull, then I will skip it, for they're not off the beaten track by my (perhaps unreasonably high) standards. If I reach a beauty spot clearly only visited by locals, then I know I have succeeded. Incidentally, when I did research Þjórsárver or Guðlaugstungur og Álfgeirstungur I found mostly Icelandic language pages, which is a good sign; and the visitor book at Arnarfellsbrekka in Þjórsárver had mostly Icelandic language entries. An outdoor destination in Iceland where >75% of the visitors are Icelanders is very rare indeed (and it's also exceptionally beautiful there).
Although I don't vacation in cities, a similar principle should work for destinations within cities. What interesting destinations do the locals know about, that visitors have not yet discovered? That may be where you want to be. Few people try this, so I'm not too worried about it becoming self-defeating.
Somewhat related question on Outdoors.SE: https://outdoors.stackexchange.com/q/4187/566