I can only judge how likely it is to be slippery. These are my experiences from doing canyoning:
Before judging the rock I judge my shoes. E.g. almost everything would be slippery in sandals. Hiking boots are better, at least not everything is slippery. Best are my canyoning boots. I usually judge slippery as slippery for these canyoning boots.
Green surfaces mean there is algae growing. Some algae is brown, but if you are close enough you can detect them as algae nevertheless. This almost always slippery.
Then I judge the type of rock. Examples: Limestone is more often polished smooth than other types. Granite is very often non-slippery. One can ponder this when walking to the waterfalls, not even seeing them.
Considering the weather a few days ago makes sense, too. Heavy rainfall can wash away slippery films. Really dry weather can dry those out, but can also lead to slippery dust on the rocks. Everything between means, there is somewhere something slippery.
At the waterfall I compare rock that is always dry to rock slightly sprayed by water and to rock directly in the water.
Usually dry rock is non-slippery. If everything looks the same it is very likely that it's not slippery. Still I am careful; there are exceptions you can not see.
If there are heavy differences like in your picture, it is very likely that there are slippery surfaces. I would suspect the lighter parts are slippery there.
As a rule of thumb the rock wetted by the spray is very often slippery. Where there is much water, slippery stuff is often washed away, but so are probably you.
Last but not least, the slope of the stone you tread on makes it more or less likely for you to slip.