You will have to provide the parameter value for @employer_id
IF (select count(*) from employer_tb where employer_id = @employer_id)=1
BEGIN PRINT 'true' END
ELSE
BEGIN PRINT 'false' END
If in your logic an employee id can appear more than once, modify the code above as:
(select count(*) from employer_tb where employer_id = @employer_id)>=1
Hope this helps