In UXF #26, Mystique disguises herself as Psylocke and sleeps with Fantomex. Despite having sex with her, Fantomex reveals that he noticed that Mystique's yellow eye flecks were visible in her disguise as Psylocke.
This raises 2 crucial questions:
1. Who's looking at Betsy's eyes?
2. More importantly, are you telling me that Iceman, Gambit, and Wolvie all overlooked that critical "tell"?