So my motivation for wanting to check the Active View was this: I have a rule which gets the Flat Pattern extents, and the lines of code which do so (SheetMetal.FlatExtentsLength and SheetMetal.FlatExtentsWidth) automatically create the Flat Pattern and switch to it if it didn't already exist. However, I wanted the Part to be in the same View after runnng the rule that it was in before running the rule.
I thought the way to do that was to check if the Flat Pattern was the active view before running my code, and if so then leave it after capturing the extents, but if not then switch back to the Folded Part view after capturing the extents.
However, I didn't think about this fact: My code will switch the active view to the Flat Pattern if and only if the Flat Pattern didn't already exist before running my code. So I don't actually have to know if the Flat Pattern view was active; I can simply check if it exists prior to running my code. If it does, my active view will not be switched, so that's that. But if it does not exist, I can be sure my active view will be switched to the Flat Pattern, and I'll need to switch it back.
So, I just added these lines to my rule:
If oCompDef.HasFlatPattern = False Then _CreatedFP = True
'Capture Extents (these will switch the active view to the Flat Pattern if and only if it doesn't already exist)
EX=SheetMetal.FlatExtentsLength
EY=SheetMetal.FlatExtentsWidth
'Do stuff with EX and EY
If _CreatedFP = True Then oCompDef.FlatPattern.ExitEdit()
So once again, I was just overcomplicating things 
It might be nice if there were a way to check the current active view for Sheet Metal parts, but in this case I was able to do what I need without it.