TRWTF here is that you can't do a DEBIT CNP (Card Not Present) transaction... Unless... DON'T TELL ME YOU ARE ASKING FOR THEIR PIN...
ANY CNP transaction should end up processing as CREDIT... A customer can use a debit card, but it will ride the credit rails instead of routing through the debit networks... Meaning the fees from the issuer (Visa/MC) should be consistent, whether the card is technically DEBIT or CREDIT.
To answer the question explicitly, there is no way for you to determine credit/debit based on the card number alone.
@RaceProUK, unless you have a list of BINs that you can lookup against and cross to Debit/Credit, that won't help. I doubt you will find this list of BINs...