There can be several methods of validating a Leap year (which has 366 days) but using ISDATE function is one of the methods.
Consider the following code
declare @year smallint
select isdate(cast(@year as char(4))+'0229') as is_leap_year
The result is 1
The idea is to create a date value of 29th February of that year and see if it is a valid date using ISDATE function. If ISDATE returns 1 then it is a Leap Year if it returns 0, it is not a Leap Year