There are many different ways. For example, if there is a dominant MO transition for the studied excited state, you can simply use main function 0 of Multiwfn to visualize the two oribtals to assign the transition character. If there is no dominant MO transition, you can use Multiwfn to obtain natural transition orbitals (NTO) and visualize the NTOs to determine transition character, see Section 4.18.6 of Multiwfn manual for example. Also hole-electron analysis illustrated in Section 4.18.1 is quite useful, and it is much more universal than NTO analysis.If you don't understand which electron excitations notably contribute to the absorption peaks and thus should be subjected to above analyses, when you use Multiwfn to plot UV-Vis spectrum, you can decompose the spectral curve to individual contributions from various electron excitations, see Section 4.11.2 of Multiwfn manual for example.
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