I mean you can see what damage is autoattack from boss and what is bleeding, it seems to alternate and should be fine. The autoattack have the physical blue sword icon infront of the damage numbers.
Bosses will immediately do an autoattack after finish casting, so it kinda works out I think. If you are unlucky you get a tick immediately after it being applied, if I remember how their system works correctly.
You can also never rely on the HP bars in the party HUD or the HP bars floating around, I think the only reliable indicator seems to be targeting a party member, the target bar is reliable. The other can have "delay" and inaccuracies.
(But haven't done this boss yet, not sure if there is more to it.)