Relevant entries from Ultimate Equipment:
Cursed items are identified like any other magic item, with one exception: unless the check made to identify the item exceeds the DC by 10 or more, the curse is not detected. If the check is not made by 10 or more, but still succeeds, all that is revealed is the item crafter's original intent. If the item is known to be cursed, the nature of the curse can be determined using the standard DC to identify the item.
These cursed items malfunction so that they either do the opposite of what the creator intended or target the user instead of someone else. The interesting point to keep in mind here is that these items aren't always bad to have—some have powerful applications once the effect is fully understood. Opposite-effect items include weapons that impose penalties on attack and damage rolls rather than bonuses. Just as a character shouldn't necessarily immediately know what the enhancement bonus of a noncursed magic item is, she shouldn't immediately know that a weapon is cursed. Once she knows, however, the item can be discarded unless it possesses some ability that compels the wielder to keep and use it. In such cases, a remove curse spell is generally needed to get rid of the item.
To sum up:
A. You use the base Spellcraft DC to identify the item, and then add 10 to the DC to find out that the item is cursed. A standard identification would reveal the item's original intent (which is the Monk's Robe description), but you'd need to properly roll the Spellcraft check to determine if it's actually cursed, which would be a DC 35; almost unheard of.
B. Your party member created the item (which means he'd know the result of the item, and therefore would be able to determine it's cursed just from that alone), and you've felt the adverse effects of the robe, which means you'd know first hand that it's cursed. So, because of this information, you simply need to identify what the curse does (in this case, it's an inversement) using the standard Spellcraft DC, and if it requires a Remove Curse spell to take off. (Chances are, it will.) The DC for identification in this case would be 25. Still tough, but fairly doable at your level.
To create magic items, spellcasters use special feats which allow them to invest time and money in an item's creation. At the end of this process, the spellcaster must make a single skill check (usually Spellcraft, but sometimes another skill) to finish the item. If an item type has multiple possible skills, you choose which skill to make the check with. The DC to create a magic item is 5 + the caster level for the item. Failing this check means that the item does not function and the materials and time are wasted. Failing this check by 5 or more results in a cursed item (see Cursed Items for more information).
The bolded part reinforces my point; like a real-world vehicle, if it simply doesn't function the problem could be just about anything, ranging all sorts of issues, even to ones that just don't make any sense. If the item doesn't function correctly, then you can tell the problem based on the differences between a correct one and an incorrect one.
Despite that point, the RAW indicates the character would know the results of his check, in that he made an item that doesn't function at all. Therefore, it should be easier to adjudicate that he made an item that doesn't function correctly, especially based on observing your reduced combat effectiveness.
As for you not knowing about the curse, there are two things to consider, and that is damage in relation to damage reduction, as well as damage comparison, starting with the latter.
Let's take a Longsword versus a Bastard Sword. Longsword is shorter, and does less damage, but is easier to wield. Bastard Sword is longer, does more damage, but requires special training to wield properly. While the other two ideals are irrelevant, the fact is you should be able to tell that a Bastard Sword has more destructive capability (in terms of raw damage) than a Longsword does based on your usage between the two; assuming equivalent proficiency, of course.
Another example is when you're making an attack against an enemy who has Damage Reduction. While you may have hit the enemy with full force, the factor that your damage is still reduced is enough of an indicator that your attack wasn't as effective as it normally should be, and the rules outright say that this is the case. I fail to see how using an item in action, and having a reduced effect (especially since before this you were actually more powerful than before) isn't much different than using an ineffective weapon against a particular enemy, in that once you've seen it in action, you would be made aware of specific factors (i.e. "This weapon/item doesn't work as well as it normally should.:)
I mean, you could try to make the DC 35 check if you feel that's what the GM wants, but since you are with someone who made the item (and therefore should know the results of said item), and you've used the item yourself and felt its adverse effects (instead of the beneficial effects that it normally should have), it should be enough to warrant the negation of the +10 to the Spellcraft DC, and if a Remove Curse spell is needed (which I will most likely say that it will be).