Not really possible to say, you have to look at what they list, and if they don’t list it, then it’s a guess (or you can contact them and ask). Inherent iron isn’t set in stone. Some places grow forages that have a very reasonable level of Fe, and some have enough that 2% of body weight in it is easily 3-4x what the horse needs. Adding iron oxide, for example, is fairly irrelevant, as it’s not very bioavailable.
Tribute Seniority for example has 125ppm Fe, no added Fe.
Triple Crown Senior has 150ppm and I happen to know they add some, but it’s really not a lot.
The difference is 11mg/lb of Fe, or 66mg over 6lb, which in the grand scheme of things doesn’t matter
That said, if I had an EMS/IR/ID horse, I WOULD look for things lower in Fe (regardless of added or not) since they may accumulate Fe inappropriately as part of their disease, but the difference of 66mg (if I was even able to feed 6lb to an EMS horse) wouldn’t bother me
SOME feeds have unnecessary amounts of iron, and because it’s so high relative to some (some!) feeds that don’t add any but list it, it’s reasonable to think they’re simply adding too much. But even then, the question is - what form is added? Sometimes it’s a form that’s not very bioavailable to the horse, but you still don’t know how much IS.