How to select true/false based on column value?

Use a CASE. I would post the specific code, but need more information than is supplied in the post - such as the data type of EntityProfile and what is usually stored in it. Something like:

CASE WHEN EntityProfile IS NULL THEN 'False' ELSE 'True' END

Edit - the entire SELECT statement, as per the info in the comments:

SELECT EntityID, EntityName, 
       CASE WHEN EntityProfile IS NULL THEN 'False' ELSE 'True' END AS HasProfile
FROM Entity

No LEFT JOIN necessary in this case...

You can try something like

SELECT  e.EntityId, 
        e.EntityName, 
        CASE 
            WHEN ep.EntityId IS NULL THEN 'False' 
            ELSE 'TRUE' 
        END AS HasProfile 
FROM    Entities e LEFT JOIN 
        EntityProfiles ep ON e.EntityID = ep.EntityID

Or

SELECT e.EntityId, 
        e.EntityName, 
        CASE 
            WHEN e.EntityProfile IS NULL THEN 'False' 
            ELSE 'TRUE' 
        END AS HasProfile 

FROM    Entities e