Found it. Had a look on the vdp proxy and in /usr/local/avamarclient/var you can find some files telling you which CBT blocks are being used:
---
2016-02-11T19:00:49.683Z avtar Info <10695>: Processing 45 changeblocks for "virtdisk-flat.vmdk" (85,899,345,920 bytes)
2016-02-11T19:00:49.684Z avtar Info <17957>: Initiating CBT backup to Data Domain using Virtual Synthesis.
2016-02-11T19:00:49.696Z avtar Info <18120>: DDR trace is enabled.
2016-02-11T19:00:49.696Z avtar Info <15091>: Getting the previous backup's /cur/53214dc7d533138b31e469a50ae44c72da2e3e37/1D1643606E3F22E/ddr_file.xml from DD system instead of Avamar server.
2016-02-11T19:00:49.696Z avtar Info <15261>: Getting '/cur/53214dc7d533138b31e469a50ae44c72da2e3e37/1D1643606E3F22E/ddr_files.xml' stored on DD system from previous backup
2016-02-11T19:00:49.696Z avtar Info <19156>: - Establishing a connection to the Data Domain system with encryption (Connection mode: A:3 E:2).
2016-02-11T19:00:50.133Z avtar Info <15260>: Opening previous backup '/cur/53214dc7d533138b31e469a50ae44c72da2e3e37/1D1643606E3F22E' to synthesize new backup '/STAGING/53214dc7d533138b31e469a50ae44c72da2e3e37/BACKUP-C5D28622765E865C7B2CFF1AF45453F385592B6E/125D1594E53971850EF79345A81B2499CD97A992'
2016-02-11T19:00:50.133Z avtar Info <16398>: Opened container 'VMFiles/1/virtdisk-flat.vmdk' of type 'raw' for stream 13 and opened previous container 13 from base backup '1D1643606E3F22E' with handle 1 for changed block backup using synthesis
2016-02-11T19:00:50.468Z avtar Info <10698>: - Changeblock #5 offset=11341135872, bytes=262144
2016-02-11T19:00:52.374Z avtar Info <10698>: - Changeblock #10 offset=12353273856, bytes=589824
2016-02-11T19:00:53.681Z avtar Info <10698>: - Changeblock #15 offset=22182625280, bytes=131072
2016-02-11T19:00:53.810Z avtar Info <10698>: - Changeblock #20 offset=22218145792, bytes=196608
2016-02-11T19:00:53.871Z avtar Info <10698>: - Changeblock #25 offset=24330108928, bytes=65536
2016-02-11T19:00:54.091Z avtar Info <10698>: - Changeblock #30 offset=28433383424, bytes=65536
2016-02-11T19:00:57.212Z avtar Info <10698>: - Changeblock #35 offset=32920174592, bytes=131072
2016-02-11T19:00:57.825Z avtar Info <10698>: - Changeblock #40 offset=39363805184, bytes=65536
2016-02-11T19:00:58.082Z avtar Info <10698>: - Changeblock #45 offset=52247789568, bytes=196608
2016-02-11T19:01:39.649Z avtar Info <0000>: - avamar-1443466791/125D1594E53971850EF79345A81B2499CD97A992: 173.2 MB (0.211%) was scanned and written in 3.666 seconds (47.23 MBPS), 79.83 GB (99.789%) was synthesized on the DD system in 4.094 seconds (19.50 GBPS)
2016-02-11T19:01:39.649Z avtar Info <0000>: - avamar-1443466791/125D1594E53971850EF79345A81B2499CD97A992: 0 bytes (0.212%) new data backed up with network load of 174.6 MB (0.213%)
2016-02-11T19:01:39.660Z avtar Info <15092>: Container final statistics:
container file name: 125D1594E53971850EF79345A81B2499CD97A992
total seg count: 28,366
redundant seg count: 6,923
---
But for some other VM, it can't find a reference so doing a full backup (again)
2016-02-11T14:31:49.743Z avvcbimage Info <11986>: Changed block tracking is engaged for this VM
2016-02-11T14:31:49.743Z avvcbimage Info <11988>: A reference to a valid prior backup is not available so this will be a full level zero backup.
2016-02-11T14:31:49.757Z avvcbimage Info <19549>: metadata tmp dir: /usr/local/avamarclient/var/vmware/1/temp
Checking why it can't find any previous full backup for this one.
