When a Sha-zenJin wishes to, they can perceive the very fabric of magic itself. Although each DM will have their own idea about how to portray the Loom, the over arching concept is that reality is made up of an infinite number of magical strands, or skeins that connect everything to everything else.  The strands of Multicolored light stretch, bend and intersect creating patterns that make up everything.  This includes things that are hidden by spells, magical effects, wards, and spirits. The Sha-zenJin can detect these things passively or actively.
      Spells in the Loom:  Spells usually appear as geometric constructs that rest on the target or originating source.  As you might expect, these constructs have varying levels of complexity and forms based on many variables. 
       Spirits in the Loom:  Spirits that linger close to the material plane can also be perceived in the loom. They usually appear as they did, in life, or as they truly are.  For the purposes of LOS, spirits are solid.   
       Wards in the Loom: wards and abjurations usually appear as flattened patterns and circles that cover the protected area. For example, a protection from evil spell may appear as a patterned circle that appears on the target's chest.  
       Magical effects in the Loom:   magical effects in the Loom often look similar to similar spells, but simpler and more organic in appearance.  

       Passive Loom Sense

The DM can roll for passive detection checks against a DC of 14 - spell or effect level.  If the Sha-zenJin is alert, and spends a bonus action focusing on their perception, they can make this check with advantage. A successful detection simply reveals that there is magic in use somewhere nearby and a somewhat vague direction of its possible origin. 

       Active Loom Sense

When a Sha-zenJin wishes to fully perceive the Loom, they use a free action and simply extend their spirit a little into Loom Space. By doing this they are able to perceive and (with training) manipulate skeins of the Loom.  Doing this allows the user to automatically spot spells, wards, magical effects, and spirits present within its detection radius. However,  if those things are not in the users LOS, (like hiding around a corner) the perceiver knows of its presence and general location only, much like when passively detecting it. This is because Loom Sense does not replace the user's senses, but adds to it. So, a PC perceiving the Loom still sees the physical world, with the Loom overlaying and within it.  Active use of loom sense also confers darkvision upon the user with a radius of 20' + 5' per user's level. 

      It should be mentioned here that utilizing loom sense has some inherent dangers associated with it.  There are some nasty entities that exist primarily or solely in Loom Space. When a Sha-zenJin shifts their spirit into Loom space, it makes the Loom ripple in a way that can sometimes attract some unpleasant beings.  Generally, the longer a Sha-zenJin lingers in the loom, the more likely they are to attract or annoy something living there.  The DM should decide how likely those encounters are and what exactly they might entail, but for some randomness, just refer to the Loom random encounter table.