The main difference is micro-texture. You cannot differentiate between both from a picture or a view, you need to go to the field and:
- Break the rock with your hammer to observe
the rock in fresh-cut.
- Add a bit of water to remove dust and correctly observe the colour and texture of the rock.
- Finally use a magnifying glass to determine what the rock is made
You should differentiate them as sandstones are always composed of grains. The texture is always grain-supported.
Limestones are composed of calcite. The texture use to be different, but it migth have also carbonated grains.
Rebooted Dunham classification of carbonates. Image from: commons.wikimedia.org
You can differentiate them in all cases unless you have a grainstone limestone with sand-grade grains (63μm-2mm). Then you need to identify the carbonated grains that can be fossils or oolites.
Oolites. Image from: commons.wikimedia.org
Fossils are clearly differenciated from sandstone grains with a magnifying glass, and oolites have a concentrical structure as shown in the picture.
Alternatively you can use dissolved hydrochloric acid or vinegar. As limestones are composed by calcite you will see a reaction, but you could also see the reaction on a sandstone if it has carbonated components.
Once you have determined the composition of a rock bed you can do the cartography at the lab with a stereoscope and aerial pictures or do it at the field again painting the limits on your map.