Usually, if I find something good in a fic I write in the comment 'I might use this', (more than often it is just a word or an expression). Later on I have no idea where or when it came from, but at least the author knows that s/he is quoted somewhere.
I would like at least that if it were me who was being the source of inspiration.
Most often I get a reply telling me that it's ok to use, and that they like that I like it. I haven't gotten a 'no' yet.

When it comes to music, movies and 'real' fiction I always say what it is, even link to it if possible.

But then again I have not seen if I have been 'copied' anywhere. Then I guess it would depend on the context.