Depends how long they have had it.Most stomach cancers don't start "creating" symptoms/problems until someone has had it for roughly 2 years apparently.
In some cases,the "lucky" ones the Cancer is found whilst undergoing investigations for something else.
In my Dads case,he was on Warfarin (to thin his blood) and he just suddenly started vomiting blood whilst on holiday(maybe it was the rich food and increased alcohol intake)
The hospital presumed it was an ulcer,and put a camera down,took some biopsies,and he had Stomach Cancer.
When he came home,he went to University College Hospital in London,where they have a team of specialists in Stomach Cancer,he had part of his stomach removed,and then went on to have Chemotherapy.
We are told he was very very lucky to have "found" it so early,and apparently,"Fingers-crossed" he is cured.This was in August 2006,had the op in December 2006,today he eats normally and is healthy.He is followed up regularly to check for recurrence.HTH