Determining if an entity exists in a database via a stored procedure
I may be pointing out the obvious here, but the ValidateEntity Method returns a string. I would expect it to return a Boolean. 
There are a number of one and two letter variable names. We've not been restricted in the size of names for many years now. It would be good to give them more descriptive names and we can take advantage of C#'s case sensitivity. 

da = adapter
ds = dataSet (but perhaps something even more meaningful)
dt = table or resultNames

If you return a bool instead of a string, you could directly return here instead of using an if statement. 

if (dt.Rows[0]["Valid"].ToString()=="Y") { return "True"; }



Becomes

return (dt.Rows[0]["Valid"].ToString() == "Y");