Right... i really REALLY hope that this doesnt bleed over in to the X-Comics too much. Sure Colosus could be a skrull, because the whole body swapping story was a bit random but theres some REALLY big problems with making the X-Men members skrulls.
First and most arguable reason, the X-Men dont just have tecnology to scan people, they have ridiculously sophisticated technolog from the Shi'Ar, who have fought Skrull before and would be well aware of any techniques to find them with scanners.
Next we have the irrefutable reasons... the X-Men have had the 3 most powerful telepaths IN THE WORLD in their midst over the last few years.
If Emma is a Skrull, that would mean that Phoenix.. an all powerful telepathic/telekinetic being, stripped down her mind, pieced together her broken body, repaired her, brought her back to life, scanned her entire mind, and just didn't happen to notice that she happened to be a Skrull. This is daft. If Emma has been a skrull this whole time it will frankly be a pretty embarassing move on Marvels part.