How Jed knows that your file won't sew is a little beyond me maybe you're colleagues? Otherwise it isn't always correct to assume, but what may be occurring is that in many cases people have automatically sew turned on in their import setting so that if it has failed then you know that it won't sew.
Now the case with IGES imports is usually that surfaces sometimes just won't trim to their edges in may cases. It is also usual that it occurs for regular surface types more than the free form shapes so it will likely be on cylinders and flat surfaces. There will almost always be just enough surface to trim but it may help to slightly extend the original, and it will very often make sense to extend that neighbor surfaces and trim to a new intersection.
How you find these things is to attempt a sew and look at the results. The bext tool for this is examine geometry and you should fix and repeat checking until you get it right.
The main thing is that there may only be a few surfaces that are affected and even on a bigger part you may find a dozen of so little problems. These thing rarely affect anything like the majority of the part and I have managed to quickly fix many in the past. Admittedly where assemblies of larger products are involved you have to manage how much time that you waste on these things. However you should always be able to fix them with a little patience and not as much time as you might think.
If there are two over the top of one another then it will usually only sew in one, but if the surfaces overlap it will show up under examine geometry as a face-face intersection. When in the first case it does only sew the one a warning message will alert you to that. Looking at the object count under layers and keeping an eye on the status bar as you work will also help.
Another good tip while you're just looking for edges that mismatch is to set your examine geometry to "sheet boundaries" only so that it will very quickly find and display the results.
