How to distinguish between a bed and a layer through outcrop survey and thin section observation?
Thin section will not assist. The evidence is in field observation. Metamorphism and deformation can produce layering, but it is generally at small scale, such as the gneissic lamination found in granite gneiss. Have a look for sedimentary structures, such as graded bedding in siliciclastic metasediments - graded bedding is defined by particle grainsize, but is also compositional, with fine grained material containing more clay and coarse grained material containing more sand (quartz and feldspar). Layering at a scale of about 50 mm or more is likely to be original; bedding in metasediments; aplite dykes, schlieren in granite. Deformation can transpose bedding, shredding the original bedding so that adjacent beds might young in opposite directions. But it is still bedding.
You must have seen very many outcrops different in their sedimentary lithology (siliciclastics (including pyroclastics), carbonate and even evaporites placed in order of their increasing solubility and susceptibility to mobilization), consider the textures and structures be it bedded, domed, or vein-like. Then you have your platform to build upon your edifice and you can move to results of rock transformation. Look at all kinds of rock transformation from low- to high-temperature as well as low- to high pressure (regional metamorphic contact metamorphic/metasomatic) and do not cast aside mechanical deformation or kinematic processes (structural geology). Start off with all lithologies called metapelitic, metapsammitic..to meta-evaporites because in these outcrops you can still see the sedimentary realm well preserved what you no longer can expect in, e.g., a para-amphibolite. And here you can see what metamorphic mobilization looks like and you can distinguish it from anatectic processes at high T prior to forming a migmatitic and granitoid rock. If you are beginner read textbooks of sedimentology, metamorphic petrology and structural geology rife with pictures and sketches and parallel to this approach taken go into the field (there is no geology without field geology). Look, look , look, describe, describe think it over and interpret. Never the other way round !
A good geologist is a geologist who has seen many sites different in scale and type.
There does not exist a philosopher’s stone, it will be hard work. But you will reap the benefits from all those efforts taken.
I wish you "Glückauf" and good luck
HGD
Metamorphism and deformation can produce layer at small scale but to confirm , one must search for sedimentary structure
At the outcrop or sample scale, metamorphism can completely erase the original sedimentary structure so that another "layered" texture appears, this being the phenomenon of transposition. Metamorphism is generally accompanied by deformations and in this case it must be observed very carefully, in outcrop, in the sample, or even at the scale of thin sections, if there are relict deformational structures within the same "layer", the most commons and obvious being the "hooks". The presence of these "hooks" indicates deformations or metamorphic (re)folding and a transposition of the original sedimentary texture.
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