There are fancier ways to do this, but you can query the table in one of two ways.
SELECT * FROM w3t_Subscribe WHERE S_Board='boardname'
OR
SELECT * FROM w3t_Subscribe WHERE S_Uid='user#'
Replace boardname with your board keyword, and replace user# with the user you want to query.
You could probably stick a COUNT() function in there, but the above will give you the raw results.