Its not far off from looking at a circuit board and recognizing layout patterns and chip shapes to get a feel for whats going on. e.g. Larger chips are usually

Its not far off from looking at a circuit board and recognizing layout patterns and chip shapes to get a feel for whats going on. e.g. Larger chips are usually of interest. A larger chip with one or more little rectangular chips next to or surrounding it is likely the CPU and RAM/Flash/ROM. Smaller chips are interface or power regulators depending on their proximity to ports and other components. Groups of inductors, capacitors and transistors are likely power supplies. It all comes with experience.