The ALT key does not have an ASCII value: it is a modifier key and doesn't not affect ASCII values.
If you want to know if the ALT key is pressed, use Control.Keys in Winforms:
if ((Control.ModifierKeys & Keys.Alt) != 0)
{
MessageBox.Show("AltDown");
}
In web, you can't tell at all - it does not leave the browser (and the end equipment may not even have an ALT key).