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Asking a trans person if they have had "a sex change operation" is not the right terminology.
There are a variety of gender-affirming surgeries a trans person may choose to have in their lifetime.
These include top surgery and bottom surgery. Top surgery refers to changing the chest to augment breasts or remove them.
Bottom surgery is a colloquial phrase that came mean a number of different procedures, including phalloplasty or metoidioplasty, which both create a penis for people with vaginas, or a vaginoplasty, which creates a vagina for a person with a penis.
While people typically mean bottom surgery when they ask if someone has had "the" surgery, many trans people do not get bottom surgery or any kind of gender-affirming procedure.
A transgender person's medical history says nothing about their transness, so it's best to not ask unless a person offers that kind of information.