Intellego Vim spell guidelines mention this in the (badly layed-out) text, though not in the table of effects (emphasis mine):
Magic items cannot be investiged with simple magic spells. Investigation requires that you spend at least one season in the laboratory to discover what magic an enchanted item contains. A spell will suffice to tell you that an item is magical, however, and may give you some idea of its powers.
(ArM 5th edition, p.158, Intellego Vim Guidelines sidebar, top right column)
What I take from this is that the general guidelines for detecting magic apply to enchantments on an item as well, though it would take a different spell than the one used for active magic. So PeCo 30, a sixth magnitude spell, would be a base level 3 effect (same sidebar), modified with +1 (Touch) and +2 to discern technique and form (based on the lv40 spell Sight of the Active Magics) for a total of lv10 - not bad, but we're not finished yet.
If we want it to be able to detect any magnitude, we'll start with the lv5 effect. Also, Sight of the Active Magics only detects Form/Technique, not magnitude, so we'll put another +1 in there, pushing us up to lv.25 (Base 5, + 1 Touch, + 2 Techniques and Forms, +1 Magnitude).
That's nice, but judging by similar mechanics for investigating enchantments (p.100), we see that multiple powers are usually detected separately, and an Instantaneous spell probably won't be able to do that.
So our final version would be able to slowly read all powers of an item, in ascending order of power, with their techniques, forms and magnitudes: Base 5, +1 Touch, +1 Conc, +2 Techniques and Forms, +1 Magnitudes, for a total of Intellego Vim 30.