In a Pathfinder campaign I'm playing a medium level rogue with one level in wizard who is impersonating a stronger wizard. In order to pull this deception off I'm trying to procure a magic item like a Staff of Power to help me fake it. My question is, when using the magic item to cast spells, is it obvious that the source of the spell is the magic item or can it look like it's coming from my character?
Magic items don't cast spells; they grant the user the ability to cast the spells contained within them.
Spell Trigger: Spell trigger activation is similar to spell completion, but it's even simpler. No gestures or spell finishing is needed, just a special knowledge of spellcasting that an appropriate character would know, and a single word that must be spoken. Spell trigger items can be used by anyone whose class can cast the corresponding spell. This is the case even for a character who can't actually cast spells, such as a 3rd-level paladin. The user must still determine what spell is stored in the item before she can activate it. Activating a spell trigger item is a standard action and does not provoke attacks of opportunity. (PRD, Magic Items)
You are always the "source" of the spell, so even though it is the Staff of Power that grants you the ability to cast the spell, you are still the one doing the casting. And since you have a level of wizard, there are no other requirements for you to cast those spells from the staff at the lowest possible CL for each spell contained within.
However, keep in mind that anyone who knows what a Staff of Power is will likely know what spells it can allow the wielder to cast, and may still see through your ruse. Your GM may optionally allow you to make a Bluff check to avoid the consequences of this.
Spells cast using a staff may originate from the staff's wielder, but the staff must still be held forth to be activated
Magic Items on Staves says, "To activate a staff, a character must hold it forth in at least one hand (or whatever passes for a hand, for nonhumanoid creatures)." So, unless the GM rules otherwise, holding forth the staff of power is necessary to activate its magical powers.
Wands says the much the same thing but adds that a wand's activator must "point [the wand] in the general direction of the target or area," so a staff needs to be held forth but—unlike a wand—probably need not be held forth in the general direction of the effect the magical staff creates! This may make the staff of power-wielding rogue/wizard's high-level wizard disguise easier to pull off.