UPDATE
Guid.TryParse is available in .NET 4.0
END UPDATE
Obviously there is no public GUID.TryParse() in .NET CLR
IsGuid implemented as extension method for string...
public static bool IsGuid(this string stringValue)
{
string guidPattern = @"[a-fA-F0-9]{8}(\-[a-fA-F0-9]{4}){3}\-[a-fA-F0-9]{12}";
if(string.IsNullOrEmpty(stringValue))
return false;
Regex guidRegEx = new Regex(guidPattern);
return guidRegEx.IsMatch(stringValue);
}