How to determine if a decimal/double is an integer?

There are any number of ways to do this. For example:

double d = 5.0;
bool isInt = d == (int)d;

You can also use modulo.

double d = 5.0;
bool isInt = d % 1 == 0;

For floating point numbers, n % 1 == 0 is typically the way to check if there is anything past the decimal point.

public static void Main (string[] args)
{
    decimal d = 3.1M;
    Console.WriteLine((d % 1) == 0);
    d = 3.0M;
    Console.WriteLine((d % 1) == 0);
}

Output:

False
True

Update: As @Adrian Lopez mentioned below, comparison with a small value epsilon will discard floating-point computation mis-calculations. Since the question is about double values, below will be a more floating-point calculation proof answer:

Math.Abs(d % 1) <= (Double.Epsilon * 100)