I was wondering if it's possible to do something like this (which doesn't work):
select cast( (exists(select * from theTable where theColumn like 'theValue%') as bit)
No, you'll have to use a workaround.
If you must return a conditional bit 0/1 another way is to:
SELECT CAST( CASE WHEN EXISTS(SELECT * FROM theTable where theColumn like 'theValue%') THEN 1 ELSE 0 END AS BIT)