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.
How to spot them:
If you go to the mountains on SE, were golems spawn more frequently, you can easily tell golems apart from rocks as they look SANDY compared to the grey or shiny metal rocks.
In the dunes or else where the golems have better camouflage, apart from being more textured, either use a Parasaur or spyglass to confirm your suspicions.
Also if you see a rock on the horizon ‘Fall’ like creatures spawn, then take it as red it’s a golem.
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.