It's my understanding that when a PC finds a magic item, they need to use the Identify Magic exploration activity to determine what it does and how to activate it.
Page 283 of the Core Rulebook says that Arcana, Religion, Occultism, or Nature should be used depending on the tradition of the magic the PC is trying to identify. However, the magic items listed in Chapter 11 (Crafting & Treasure) don't include a tradition – they only have a school trait (e.g. Conjuration).
I wondered if maybe Identify Magic isn't meant to be used on magic item treasure, but page 238 does mention using it for identifying items, along with effects and locations.
How should a GM decide which skill to have a PC roll against to identify a magic item?