When an artist specifically claims another as an influence, or when this influence is well-known (or obvious).

For an example, the band Frayle on their website says:

"Drawing inspiration from bands like Sleep, Portishead, Beastwars, Blonde Redhead, Kyuss, Massive Attack, Down, Chelsea Wolfe, and many more, Frayle exists at the intersection of doom and dream pop."

 Wikipedia is often listing influences too, for example, they have a section dedicated to influences of Joy Division: among others they list the Doors, Iggy Pop, David Bowie, Kraftwerk, the Velvet Underground.