How do geologist know what they are looking at really is what they say it is?

Because the depositional environment is constantly changing, despite what Faith seems to think. What we see is a combination of highly organic swamps with little clastic input (to make nice coal seams) and low gradient streams with overbank deposits (mudstone) and channel deposits (sandstone) and often times, volcanic ash (look up 'tonstein' sometime).

In drilling coal fields, you can find three different types of sandstone. There are the sand bars that indicate streams, and there are transgressive beach sands, and regressive beach sands. They all look different in shape and continuity.Here is a huge sand bar overlying a coal seam in New Mexico. You can tell by the shape (kind of a wedge) and the presence of large-scale cross-bedding.Here is a section of drill core from a thick transgressive sandstone in a coal region which drilled directly through some kind of a shell fossil which is right where one would expect it to be: Such transgressive sandstones tend to be thicker and with greater continuity across a basin.

Well, that could fill a book.Basically, you've got various grades of coal which reflect the degree of compaction and thermal history (metamorphism). Anthracite is the highest grade of coal. (Coal - Wikipedia)... from peat to lignite, to bituminous to anthracite and eventually to graphite.

Just to elaborate a little on Pressie's responses... Yes, in fact a child could recognize the differences. However, there is a whole science behind coal analysis at the analytical level. The degree of process is the most important factor. But again, it's not just simple burial, but other factors can contribute such as deformation. They can, but the primary difference is in the degree of post-burial processes.When I went to school, there were entire laboratories dedicated to the detailed study of coal and numerous PhD's were cranked out over the years. This is not a trivial topic no matter how YECs present it.

Any time you can accumulate a pure organic sediment, you can ultimate end up with a coal bed.Here is a peat deposit in the bogs of Northern Ireland. There is almost no input of clastic material at this lcoation, just an ages long build up of organic debris. Even the streams in this immediate area have no rocks, sand, boulders, etc.Just dead grasses, some animals and (once upon a time) trees and roots.This material is, even today, harvested and used as fuel. In fact, that's why scotch tastes the smokey way it does.Interestingly, there is sheep skull in the picture. While it is younger than the peat itself, geologically speaking, it is of the same age. With very little imagination, one could see it being buried eventually and become part of the geological record. However, since we see erosion occurring here, it's not too likely in this case.Certainly, you can imagine other depositional settings, such as swamps or ponds. I understand that there are even some Precambrian coal deposits, though I'm not really sure how they formed, but basically, it's just 'pure' organic carbon residue.

As a general rule.

Ya got that right!Some things don't change, no matter which hemisphere.

Having been fooled by nature too many times to count, I'd say, not a whole lot. Certainly these are boulders of thinly bedded material, partially covered by transported (see the small ripples) sand(?), of a different composition. Possibly mud cracks are present, but without direct observation it could be some other things. The best possibility is water lain, fine sand and silt or some kind of eolian deposit (possible large-scale cross-beds on the left side of image). But that's pretty uncertain.To me, all of the Mars pictures are 'flat', without a lot of definition. I'd like to see colors, too.