What are the differences in terms of appearance between sandstone and limestone? On the field, I find it very difficult to differentiate the two types of rocks just by looking at them.I know that you should do some analysis of the sample before any conclusions. But, for example in structural geology, I have to draw a geologic cross section and differentiate sedimentary layers.

By looking at pictures I find it very hard to tell whether the rock is limestone or sandstone.
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
of.




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.