It'd certainly be a lot wiser for any accuser to show evidence right away, but I suppose they could've just forgotten. L'homme_Arbre has an interesting point, though - it might be worth looking into.

I can't blame you for not wanting to check all the fics for copied lines. You might have an easier time of it if you take some of the more distinctive lines and Google them, restricting the search to FF.N - you'll get a lot of rubbish links, but you should find out if there are any direct copies quite quickly.

You might want to try messaging the accuser and get some more details about it. If they talk to you, you'll get the information you need, and if they avoid you or start being vauge, that's generally a sign that the attack is baseless and they're being bitter.

However, I have my own theory about the matter. It's a (sort of) old story. Two friends start writing a story togeher, using one of their pre-established accounts. They have a falling out, maybe over the story, but one of them keeps writing it, using material they both devised. One accuses the other of plagiarism, they bicker, and generally, the story gets dropped out of petulance. It would't be the first time, and it seems fairly likely.

That's just what it sounds like to me. However, you should still keep checking things out. Plagiarism is one of the most serious accusations you can make on a site like FF, and if the author you like is lying about their work, you really should keep looking into it.