Whenever somebody asks me to draw a line between inspiration and rip-off, I can’t really do much better but send them this chart from Steal Like An Artist. It’s a kind of graphic summary of what T.S. Eliot said in The Sacred Wood (which also serves as an epigraph for the book):
Immature poets imitate; mature poets steal; bad poets deface what they take, and good poets make it into something better, or at least something different. The good poet welds his theft into a whole of feeling which is unique, utterly different from that from which it was torn; the bad poet throws it into something which has no cohesion.
In other words, despite the common saying, imitation is not flattery. It’s transformation that is flattery: taking what you’ve stolen and turning it into something new.
I don’t think whether something is good or bad theft is really that complicated. If it feels cheap or wrong to you, it probably is. I advocate an “elevator gut check” for one’s own work: If you met the artist you’re stealing from in a stalled elevator, would they shake your hand or punch you in the face?