if my Eidolon casts detect magic, is this a sense that is shared with bond senses, and am I then able to use my spellcraft to identify it if my Eidolon has 3 rounds to inspect the item?
Detect magic doesn't let you see auras, it lets you detect them. You can actually use detect magic in complete darkness or while blind.
So, no, it wouldn't transfer through Bond Senses.
I don't think so.
Bond Senses (Su): Starting at 2nd level, a summoner can, as a standard action, share the senses of his eidolon, hearing, seeing, smelling, tasting, and touching everything the eidolon does. He can use this ability a number of rounds per day equal to his summoner level. There is no range to this effect, but the eidolon and the summoner must be on the same plane. The summoner can end this effect as a free action.
But Detect Magic isn't one of those senses. I think it involves sight to a degree, but wouldn't transfer through the bond.
I think it's more like magical radar.
yeah, this is two-part me trying to figure out how detect magic even works. It doesn't say anywhere it is a sense, so i was inclined to say it didn't work that way, but at the same time wouldn't I be aware of anything my Eidolon is able to detect? it's weird.
In any case -- Wouldn't it make more sense for the Summoner to learn to Detect Magic at will rather than spend an evolution point so that the eidolon can do it once per day?
I would use Minor Magic for a cantrip that the Summoner can't learn.