First of all... Which type is your agencyid? Do you need the extra ' signs?
Moreover the SELECT statement could look a bit different; e.g. you could use the COUNT function to count the rows matching the condition, and then check to see if this number is above 0.
It is something like...
SELECT COUNT(*) AS agencyCount FROM authusers_reg WHERE agencyid = [someID]
... and then you should be able to retrieve the value something like this...
You might have to remember that you should do some conversion, if you are comparing the value to some integer. The code you posted will not work as far as I know.
Hope it helps.