Airport Depature Tax Changes, February 1, 2007 by WSH80
As of February 1, 2007, TWO things happened at exactly the same time 1) the departure fee went to 700 baht; 2) the system of collecting the departure tax by means of a separate payment, evidenced by a receipt, which would be punched by someone just before the immigration windows, was abolished, and was replaced by having the price included in your airline ticket.At some point well prior to February 1, 2007, ticket agents and airlines began including the 700 baht departure fee in the price of the ticket, for departures out of Thailand set for on or after February 1, 2007. So if you bought a ticket in November for departure from Thailand after February 1, the fee was probably included in your ticket.
If you are departing after February 1 then the price of the departure fee should be in your ticket. If it is not, I understand that there is some interim means by which you pay it separately. I don't know how they will know if it is included or not. I guess the airline check in staff have to peruse your ticket in some detail. If it was not included, and they catch it, you will have to pay 700 baht. Eventually, in a few more months, the change will have completely worked its way through the system.