No. A Ready Action Will Not Trigger on an Enemy that is Concealed from You.
tl;dr You need to be able to perceive (not limited to sight) the event or creature that will trigger the ready action.
The Trigger for a Ready Action Needs to be Perceivable.
"I use my action to ready a ranged attack against any potentially concealed enemy that shows up from above to range attack us"
The term "concealed" in the above trigger can be removed. A concealed enemy can't be perceived and one that shows up isn't concealed. The description of ready action indicates the trigger must be perceivable:
First, you decide what perceivable circumstance will trigger your reaction. Then, you choose the action you will take in response to that trigger, or you choose to move up to your speed in response to it.
The Enemy is Perceivable
The hide doesn't beat your passive perception, then it will trigger the ready action.
The enemy is Concealed
It is hidden and beats the passive perception, then it will not trigger the ready action.
Passive Perception is Always On
.. do we assume my character is looking all around to find this potentially hidden foe?
You are always perceiving around you, and that's more than looking.
In this podcast Jeremy Crawford talks extensively about stealth. At ~11:00 he states that it is up the GM to determine if the conditions for stealth are applicable. During the interview, he repeated states that passive perception is always on, and goes into some detail about how that works for rolls.
You can attack an unseen enemy.
Attacking a hidden or unseen enemy is done at disadvantage.
From the PHB:
When you attack a target that you can’t see, you have disadvantage on the attack roll. This is true whether you’re guessing the target’s location or you’re targeting a creature you can hear but not see.