How can I check if my snapshots are incremental (using hard-links)?

Please compare the inodes of a file that definitely didn't change between two snapshots. For this open two Terminals and 'cd' into both snapshot folder.
'ls -lai' will print a list where the first column is the inode which should be equal for the same file in both snapshots if the file didn't change and the snapshots are incremental.
The third column is a counter (if the file is no directory) on how many hard-links exist for this inode. It should be >1. So if you took e.g. 3 snapshots it should be 3.

Don't be confused on the size of each snapshot. If you right click on preferences for a snapshot in a file manager and look for its size, it will looks like they are all full snapshots (not incremental). But thats not (necessary) the case.

To get the correct size of each snapshot with respect on the hard-links you can run 'du -hd1 /media/<USER>/backintime/<HOST>/<USER>/1/'

('ncdu' is a bit confusing with hard-links so I won't recommend using it)