There is another way, although not quite as iron-clad as checking the main jet size.
Remove one of the carb tops and check out the throttle slide spring and the needle.
If the slide spring has been cut - it will be pretty obvious if you compare the ends of the spring - there's a good chance that it's been re-jetted.
Now, check the needle. If it has SD-130 stamped on it, it's stock and it will have a straight-line taper.
If it's not stamped SD-130 (poss SD-110, there are two needle part numbers, one type for the inner two carbs, the other type on the outers), it will be an aftermarket needle.
If you're unlucky, it will be Dynojet needles (which are same taper as stock, just mounted higher) but odds are in the USA that you will have one of Ivan's kits installed. His needles have 5 very subtle changes in the taper along the length of the needle.
Since your bike is running perfectly, my money is on yours having Ivan's Inside, and because the airbox is stock, I suspect it's his Slip-On kit.
This should work fine with a set of Holeshot headers but not sure that Dale would agree ...