Dynamic Sql - How To Use 'if Exists' With Variable Tables..?

Hi all

In the SP below im (trying to) do some dynamic sql. As you can see the table
to use is set as a variable and the 'exec' method used to run the
sqlstatements.
My problem is that the 'if exists' method is not doing what i was hoping it
could do.
The @presql command returns somewhere between 0 or 50 rows (give and take) -
i just want the 'if exists' part to determine if the select statement
returns something or not since i then will have to update a current row - or
insert a new one.
Even if there is no rows returned, the 'if exists' command will return true
:-/

Any suggestions to a different way of approach...?

Thanks in advance :-)

######## Stored procedure start ########
[various @ variables]
....
declare @presql varchar(200)
select @presql = 'SELECT * FROM '+@CurrentDB+' where btsiteID='+
cast(@SiteID as varchar(6))+''

IF exists((@presql))
BEGIN
declare @UpdateSQL varchar(400)
set @UpdateSQL = 'UPDATE '+@CurrentDB+' SET btDate='''+cast(@FileDate as
varchar(12))+''''
exec(@UpdateSQL)
END

ELSE

BEGIN
declare @InsertSQL varchar(2000)

select @InsertSQL = 'INSERT INTO ' + @CurrentDB + '
(btDate,
btTime)
VALUES
('''+ cast(@FileDate as varchar(12)) + ''','
+ cast(@ImportTime as varchar(6)) + ')'

EXEC(@InsertSQL)
END
######## Stored procedure end ########