How to apply if condition for checking number?

Depends exactly what you are doing. Normally, this kind of check is to make sure that the string is a number, rather than "contains at least one digit".
You can check by looking at the digits...but I wouldn't.
I'd do this:

string result = System.Text.Encoding.ASCII.GetString(temp);
int value;
if (int.TryParse(result, out value))
   {
   ...
   }

That way, you confirm that it's a valid number (which can include a leading minus sign) and get the value for later use if you need it...which you probably do.