The exists query is a convenience that sets the query to only select the Id property, use a maxRows = 1 and tests if the query returns a row.
boolean userExists = new QContact() .email.equalTo("email@example.com") .exists();
select t0.id from contact t0 where t0.email = ? limit 1
We could instead write this query using findIds() and maxRows() but it is a common enough pattern to perform checks like this that we have this exists query.