How can I tell if I am doing enough damage?

[seekified]

I'm not sure which is better for 4+ mobs; Katon or Doton. I usually put down Doton first, then use Kassatsu and then Katon, but not sure if it optimal way.

As for this, you should do it the other way around. Doton is special - Kassatsu will make every tick crit for Doton's whole duration. So Katon > Kassatsu > Doton and then spamming Death Blossom is the best AoE you can do as NIN.

But yes, Stone Sky Sea is the only "official" way of finding out if you're doing decent damage or not.