Size of their tentacles are obvious. Also, their rounded ears are much longer than usual Octoling's. While their eye maskings aren't same as Inkling's but they're black unlike Octoling's purple eye rings.
Offical Splatoon art of Octoling without visor
Look at this image for ears:
https://bravebravesirbrian.deviantart.com/art/Octoling-Ears-615645126
Sorry for horrifying you of this bald Octoling model
And look at images in
aceofscarabs' post.
Their mid-transformation models are different, too.
Splatoon (note this is hacked in) From NWPlayer123's video
Splatoon 2 (from Octo Expansion trailer)
Inkling (left) and Octoling (right) from Octo Expansion trailer
Splatoon's original Inkling mid transformation (note this is male model. Girls have two long tentacles but in Splatoon 2, they're removed)
I miss it :( No eyes visible in Splatoon 2's mid transformation...
As you can see it, enemy Octoling has more rounder head and much larger tentacles. Playable Octolings have small tentacles and bumps on the sides of their heads. They're much closer to Inklings than original Octolings.
And this pic. Aren't they so cute?
Then this pic. Whoa, they look scary.
The important Sunken Scroll from Splatoon 2
"The latest research has produced startling revelations about Inkling ink! One thought to simply disappear after a while, Inkling ink is actually rapidly disintegrated by airborne microbes!"
I think this implies that Octarians unable to produce ink similar to Inklings' therefore Octolings are barred from entering Turf War and other modes. Well, nobody wants to clean up their ink.
Not even Jellyfish. What cheap. Mean corparates. Etc.