How to find out the ECDSA private key perfix version for Altcoins?
Take the private key. It's probably expressed in base-58 encoding, so you just have to translate the number into base-16. The prefix is there in hexadecimal form.

To convert a base58 number to hex, check etotheipi's code. Based on it:

b58_digits = '123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz'

def base58_to_hex(b58str):
  n = 0
  for s in b58str:
    n *= 58
    digit = b58_digits.index(s)
    n += digit
  return hex(n)