Your DM decides what "familiar" means, a description could be enough
"Familiar with" is not a defined game term, so it is up to the DM to decide what counts as familiar. In doing so, they might refer to the rules in the Teleport spell, which lists a number of different levels of familiarity, or they might not: those are not general rules for what it means to be familiar with something, those are specific definitions for it for the Teleport spell.
If the DM decides to follow Teleport's lead, then the lowest rung of familiarity that works is "Viewed once or described":
- “Viewed once or described” is a place you have
seen once, possibly using magic, or a place you
know through someone else’s description, perhaps
from a map.
For this, you do not need to have seen the place, it is sufficient that it has been described to you by someone, or you may have found a map of it.
P.S. Note that you can see a location when scrying on a creature in it. When you use the option to scry on a creature, the Scrying spell says (unchanged in this regard from it's 2014 predecessor, emphasis added):
On a failed save, the spell creates an Invisible, intangible sensor within 10 feet of the target. You can see and hear through the sensor as if you were there.
So, if you scry on the wizard Xenobus, you can see as if you were there, within 10 feet of him, and observe the location he is in. Once you have seen that location, it is a location you have seen, and you can target it directly with subsequent uses of scrying that require you to have seen a location. Or, if your DM uses the rules for familiarity from Scrying, you could use it as a vaguely familiar location for Find the Path, even if it hasn't been described to you.