Trace from the character to the lightsource if the trace reaches the light then the character is lit, else it is in shadow. I’ve seen people trace from sockets attached to the character to get a better representation than using a single trace.
To determine the quantity of light hitting the character you would calculate that based on the light’s falloff and intensity.
Won’t account for indirect lighting.
There are tutorials all over youtube for this btw, and plugins on the marketplace.