Amanda will use incremental backups as long as you have not disabled them and it will have a full backup within the specified backup cycle. Configure amanda to email you the backup reports. The statistics columns of the report indicate full and incremental backups. The Dump summary reports fill backups as 0 in the 'L' column. Incremental backups will have a value of 1 or more depending on which level of backup they are incremental from.
You can configure amanda to always do full or incremental backups for a device. This is covered in the documentation. It is best to allow amanda to schedule incremental backups according to its planning. This will result in full backups being scheduled throughout the week, with incremental backups as needed on the other days.
I have done a setup where full backups for offsite storage were run on the weekend. This was done with a second configuration configured to always do full backups and that did not record the backups. Not recording the backups allows the daily backups to schedule their incremental appropriately.
It is worthwhile to read the documentation, especially on the strategy used to schedule full vs. incremental backups.