Check weather string Contain numbers special Char etc

The Solution 1 given by Richard MacCutchan is very good.
As an alternative if you want to check whether the string contains a particular category of characters say Punctuation or Numbers etc., then the System.Text.RegularExpressions.Regex class can be considered to test as shown below:

string text = "Is this text, contains special chars?";
//The Regex pattern matches if atleast one character of the
//alternatives is found \p{P} Unicode punctuation category
//\p{N} Unicode numbers category
if(System.Text.RegularExpressions.Regex.IsMatch(text,@"\p{P}|\p{N}"))
    Console.WriteLine ("Contains special chars");
else
    Console.WriteLine ("Does not contain special chars");

//Output
//Contains special chars

You might try below,

class Program
{
    static void Main(string[] args)
    {
        string randomCharacters = "HelloWorld2090!!!@#$";
        var result = randomCharacters.Any(ch => Char.IsNumber(ch) || Char.IsPunctuation(ch) || Char.IsSymbol(ch));
        Console.WriteLine(result);
    }
}

Hope it helps :)