Hexagonal boron nitride (hBN) encapsulation has transformed the optical spectroscopy of semiconducting transition-metal dichalcogenide monolayers (MLs) by suppressing extrinsic disorder and enabling the resolution of an increasingly complex hierarchy of excitonic states. This Review provides a spectroscopy-oriented framework for understanding and identifying excitonic complexes in hBN-encapsulated Mo- and W-based MLs and their alloys. The discussion covers bright and dark excitons, Rydberg states, charged excitons, biexcitons, phonon-assisted replicas, and higher-order Fermi-sea many-body states, including hexcitons and oxcitons, with an emphasis on the experimental signatures that distinguish them. Particular attention is given to the combined use of photoluminescence and absorption-related spectroscopy, electrostatic gating, polarization analysis, excitation-power and temperature dependences, magnetic fields, and time-resolved measurements to establish microscopic assignments. The role of alloying as an additional degree of freedom for engineering the excitonic hierarchy, spin-valley configuration, and magneto-optical response is also highlighted. By organizing the rapidly expanding phenomenology around experimentally accessible fingerprints rather than individual material systems, this Review aims to provide a practical and broadly applicable guide for identifying, comparing, and engineering excitonic states in high-quality two-dimensional semiconductors.