If you are unsure if a rock is a disguised golem or not, a spyglass shows their name and level even when disguised. Also, when disguised, the rocks often move slightly as you get close, re settling onto the terrain.
All rock elementals have the same rock skin when disguised. It is a bright yellow boulder and is very noticeable against dark mountains. Keep your distance, as they can aggro on to you when you get close.