a: ',,,,,,,'  match
b: ',,,,,,#A' no match

[^,]

if (!Regex.IsMatch(yourTestString, "[^,]"))
{
    // This string does NOT contain commas
}

var text = ",,,";
Regex.IsMatch(text, @"^,+$");    // true

private void IsOnlyOneChar(string input, char c)
{
    for (var i = 0; i < input.Length; ++i)
        if (input[i] != c)
            return false;

    return true;
}

Regex.IsMatch(str, @"^,*$");