SQL: How to properly check if a record exists

Question

While reading some SQL Tuning-related documentation, I found this:

SELECT COUNT(*) :

• Counts the number of rows.
• Often is improper

Answer 1

I'm using this way:

IIF(EXISTS (SELECT TOP 1 1 
                FROM Users 
                WHERE FirstName = 'John'), 1, 0) AS DoesJohnExist