detecting and hiding ongoing spell effects on a person

[Ettina]

In our main D&D setting, I have a set of homebrewed magic items that place an ongoing adverse effect on their owner, regardless of where the item currently is. One of the characters has sought out an NPC healer (I haven't nailed down exactly what class, but something with both magical and non-magical healing skill) and is trying to figure out what's wrong with him.

I'm wondering:

How easy is it to detect an ongoing spell effect on someone?

How much could you determine about what it does?

How could the creator of the magic items make it more difficult to detect or identify the effect of the items?

I suppose I could just homebrew an anti-detection effect on these items, but I'd rather use something supported by the rules.

[noob]

By raw magical items probably do not make their wearer emit any particular aura of magic unless described otherwise or when it cast spells on the wearer.
So you just need to cast nystul magic aura on the magic item and it can look like any other item unless it is an artefact(for some reason artefacts are just magic items but either less powerful(the case of a lot of stuff like the hand of vecna and other artifacts that are actually less good than an epic magic item) and harder to break Or they are useful but only to destroy the campaign setting for example the deck of many-things and the sphere of annihilation but only if you have a well of many worlds)
But your item could just be a mighty magical item based on owning like the cape of charisma(maybe it was another item) who do not need to be carried in fact.

[Deophaun]

How easy is it to detect an ongoing spell effect on someone?
As easy as casting detect magic

How much could you determine about what it does?
Everything, provided you can hit a DC 20+spell level Spellcraft check while detecting it through detect magic. (Or, if it has actual visible effects--like causing sores--you can omit the detect magic)

How could the creator of the magic items make it more difficult to detect or identify the effect of the items?
Magic aura can hide the aura; and it's particularly difficult to defeat as you really only get the chance to beat it if you already know it's there.

[Shadowquad]

A magic aura cannot hide spell effects on people, only objects. A solution would be that the object casts a nondetection spell on its owner in addition to the other effect.

That way, onlookers could only detect the other effect with detect magic or similar spells if they succeed on a DC 16 CL check (you can increase this DC by increasing the CL of the nondetection effect, but depending on the level of your PCs, this might be enough).

[Psyren]

Mind Blank should work - figuring out what spells you have on you should count as "gaining information about the subject."