Notably, there are two spells I've found in published core ArM products that allow the caster to see Incorporeal Creatures had very high magnitudes.
Reveal the Lingering Spirit (InMe 30) (HoH:TL p.14, originally published as InMe 20 but later errata'd to InMe 30)
Reveal the Lurking Watchers (InVi 30) (HoH:S p.99)
Both of these are (Base 5, Vision +4, D: Conc +1) construction spells, with total levels of 30. Notably, these spells also require that they penetrate - meaning exceptionally high casting totals are needed to reliably detect incorporeal creatures of non-trivial might levels.
This can make situations in which an incorporeal creature has the ability to remain incorporeal and undetected - while affecting the physical world - effectively impossible to handle for magi without the relevant specializations. For example, an incorporeal, invisible ghost could be unable to be targeted by a mage - even one with Lay to Rest the Haunting Spirit or Coerce the Spirits of the Night - without an Arcane Connection. (HMRE p.78 uses a different example, but the idea of the caster needing to sense the target barring Arcane Connections or special affects like Opening the Intangible Tunnel remains).
Based on the Guidelines in RoP:M (p.111), it also seems generally significantly easier to Control a Spirit than to detect it (at least by sight, which is often needed to properly target the relevant spirit) - a Command the Airy Spirits of the [Magic] Realm with D:Conc can be made at ReVi20, for example. To get to a comparable caster level to a sight-based detection spell, we crank up the duration of said Command to up to D:Month- getting us spells along the guidelines of those possessed by Lugh-éccnaid (TtA p.105).
Second Sight is one option, though learning this in play is typically quite difficult for Magi. Even if you start with it, or can initiate it in play, the base ease factor of 6+magnitude still makes it take fairly significant investment to get it to reliable levels - although not having to deal with magic resistance is obviously a great boon. It was also noted in the Ex Miscellanea section on Sahirs as being a particularly useful skill for them, and I can see why.
As someone interested in spirit-mage concepts - I keep finding myself drawn to Necromancers and Sahir, then I reread the section on the Titanoi in HoH:S and got particularly hype - I'm curious as to what routes others have taken to help them detect/summon/banish/bind/barter with etc for spiritual entities generally. Because of the various limits of magic, detection is a necessary step 1 before all the rest, and I'm currently struggling with it a bit.