Enemies spotting flying characters

Are there any specific rules for enemies spotting flying characters? Or does anyone have any house rules they’ve used which have worked well?

One of my players has an aarakocra ranger, and he sometimes flies ahead as a scout. If he suspects there’s enemies ahead, he flies past them, not obviously circling over them etc.

A stealth check doesn’t quite feel appropriate here, as he’s not trying to ‘hide’, and indeed he can’t hide against the sky.

However, there’s no specific reason a bunch of goblins who aren’t expecting a flying player character would:

  • be alert to check the sky, in order to even see him;
  • realise they’re seeing an aarakocra at 120′ instead of a smaller bird at 60′
  • and then realise an overflying aarakocra is scouting them rather than just generally flying past, and so either shoot at him, or prepare for a ground attack.

To be clear, I’m not looking for a way to "clip his wings"; I’m fine with having a flying character. I’m just looking for a sensible and consistent way to determine if enemies will spot him, and shoot and/or prepare. So far I’ve been rolling perception checks for enemies based on whatever arbitrary DC I pick that session, and the player hasn’t been rolling anything.

Secondly, in the situation where enemies are forewarned to be alert for a flying character, what rolls might be appropriate? In this situation it’s really just down to if they spot him. But again, rolling a stealth check doesn’t seem relevant; he can’t hide in the sky. If it were on ground, players walking past an enemy would always be spotted, but should they simply spot a flyer automatically?

If there’s something in RAW which I can use, that’s ideal, otherwise any house-rules which provide a balanced and consistent mechanism are fine. If there’s something similar to a stealth check which a player can have some agency in rolling, that may be nice for the player, but that’s not essential.