No you can not tell the difference between a credit and debit card! Agreed with the beginning # and type of card - Discover (not previously mentioned) begins with a 6. The next set is the issuing bank.
JB is right, if people are using a debit and you state that you get an approval upfront, they should know that amount is being held from their account. They can not be that foolish... (well we know better
FD - Authorizations for the full amount are good IF the reservation is going to be within the hold period...(i.e. if you did an authorization on the First, and the hold is good for 14 days, the check in needs to be within that 14 days to be assured the card still has that amount of credit available) If you are doing and authorization for a booking for 6 months from then, the only good the authorization does is to verify you have a good card. So really there are 3 ways to handle this:
1. Do an authorization for $1 - just to verify that you have a good card #
2. Continue to do an authorization for the full amount each time the one drops off.
3. Take a deposit instead of an autorization.