You can detect an IOUS ancestor when you see an obvious duplicate and try to merge the two and see an error message that the says that the individuals cannot be merged at this time. There are other reasons why some duplicates cannot be merged, but this error messages is a good indicator. The current wisdom in these IOUS circumstances is to wait until the Family Tree is no longer limited by its association with new.FamilySearch.org. If you would like the latest word on that subject, I suggest watching the video posted on the RootsTech.org website of Ron Tanner's presentation. Even if you watched the class in person, you may still want to go back and watch it again.
Now, even though some of us have focused on the IOUS problem. This is not the most difficult challenge. The IOUS problem will be resolved. What is left? The part where we add new names and correct the information that is already there.
Now where is the swamp? Here is a good example.
This screenshot comes from my portion of the Family Tree. It is easy to see that the people shown are "deceased." But who are they and how are they related to me. Those red icons indicate serious data problems but they are not the only problems. You might notice all of the "Unknown Names." What is even more serious about this entire entry? I am probably not related to these people at all!!!
Let me demonstrate what I mean. Here is the screenshot showing James Stewart Earl of Moray clicking back in my portion of the pedigree.
To get from me, through my lines, back to the person shown in the Descendancy View, James Stuart 3rd Earl of Moray, you have to back through the person with red icons. Here is what the red icons say:
Now, if I correct this what will happen to the line? Will I still be related to James Stuart 3rd Earl of Moray? This would be fairly simple to answer except for the fact that the line breaks down many steps down from this person. Here is an example of where the line breaks down and we enter the swamp.
James Stewart shown here has 14 wives listed. This is just the beginning of the problems. Many of those marriages have sources!!!
He was born in either Scotland or Massachusetts. None of this has anything to do with what FamilySearch has done or intends to do. This situation can only be resolved by careful step-by-step, sourced correction of the Family Tree beginning with the first credible ancestor.
Now, it is time to get to work. Where do you start? With yourself and your own family. Document and correct all of the entries and then move back in time.