Many deviants on dA complain about art thieves and some decide to take them down themselves.
As many would agree, Art thieves are foolish for doing what they do, they would even go to the lengths of watermarking stolen work.
If you have had with art thieves, here are a few telltales signs on how to spot them and to prevent the mistake of supporting them:

1) Art Thieves usually have unoriginal usernames. e.g 'AnimeGirl707'
2) Art Thieves usually have really horrible spelling, and prefer to type in netspeak.
3) Art Thieves usually, stupidly, uploading their own art. Compare these to the stolen art.
4) They usually generate lies on how it had taken them 'hours and hours' to complete it.
5) The final sign is when they are confronted, they usually create excuses on how they made/uploaded it for/with a friend. Persist when confronting them as they usually break down at this point.

How to confront them.
1) Approach them calmly. Don't just rush into it all guns blazing.
2) Refrain from using any type of offensive language. As if you report them, they can be reported for harassment.
3) Always find the source image. As confontation is useless without evidence.
4 Always warn them of the consequences. As refusal to heed your warning is usually their downfall.
5 And most importantly, NEVER THREATEN THEM! _{you just might end up banned...}
Thanks for reading.