I don't follow this example at all. How does the PC know one of the orcs is a wizard in the first place? If it's because he's holding a staff, wearing different clothes, or whatever, then obviously the PC has already spotted and identified him.
Not everything is obvious at first glance in every encounter (at least I don't run it that way) because of... say, lighting, how bunched the orcs are, how bunched the PCs are, smoke, etc. Maybe staff wasn't the best qualifier, but I don't see why that should invalidate the
process.
I can understand if the PC knows (or more correctly, believes) there is an orc wizard among the nine as-yet-indistiguishable orcs because someone told him: "This band of orcs is led by an orc wizard," or something like that, but if the DM tells the players that they know one of the orcs is a wizard, I would certainly assume they can tell which one it is.
Yes there are many ways for the players/PCs to know/believe/think/assume that one of the orcs is a wizard, such as what you have provided. And for orcs it is is probably fairly easy in most cases. But I used the OP's example. I didn't think that it was important to have examples of how/what they knew, just the process of why Spot might be required.
I'm not sure that the process is helped by all the extras here

(and yes, I'm expanding things too far to go

advocate here):
The PCs have been whittling down orc forces for a while now. The info that the PCs have is that an orc wizard leads these orcs but they haven't found him yet.
DM: Down the corridor you see a group of orcs. (or the party has only noticed the orcs because of a wizard spell going of in their midst, or an orc has shouted, "Wizard, they are here" and someone understands Orcish). Only the first couple of orcs can be seen clearly since the corridor is only 5" wide and the lighting is not the best.
Player: If the wizard is here I want to cast MM on him (or I want to target the wizard, or who was that orc calling to?).
DM: How will you determine which one is the orc wizard (or who cast the spell, or who the orc was talking to)?
PC: Probably the one wearing different clothes, or in no armour, probably not wielding a great big axe, different face painting etc. He will most liekly be different to the other orcs somehow.
DM: Roll spot check. (player rolls high enough to spot the couple of details that are different from the other orcs), the one at the back is the only one that appears to be different to the other orcs. Some feathers around the neck and a crossbow in one hand. All the other orcs have either a battleaxe or are wielding bows.
PC: That is the one that is most likely to be the wizard I'll target him.