> Here is some other dirty tests: If I write a website that links to someone else images, I can still be held liable under copyright law. It is not good enough defense to claim that the images themselves are not sent by my web server.

That's generally not so in the United States. The leading case is Perfect 10, Inc. v. Amazon.com, Inc., 508 F.3d 1146 (9th Cir. 2007) [1]. In a couple of other countries, linkers have run into some trouble, such as Denmark an Germany, although note that in Denmark it was not found to be a copyright violation.

A good summary of this issue for the US and elsewhere is given on Wikipedia [2].

Getting back to derivative works, a useful case is Litchfield v. Spielberg, 736 F.2d 1352 (9th Cir., 1984) [3]. Relevant discussion from the court's opinion:

Litchfield argues that defendants violated section
106(2) of the Copyright Act, 17 U.S.C. Sec. 106(2)
(1982), by preparing a derivative work based on
Lokey. The district court did not address this
issue.

Section 106(2) grants the exclusive rights to
prepare a derivative work to the holder of the
copyright. 17 U.S.C. Sec. 106(2). To constitute a
violation of section 106(2) the infringing work must
incorporate in some form a portion of the
copyrighted work. Notes of the Committee on the
Judiciary, House Report No. 94-1476, reprinted in 17
U.S.C. Sec. 106(2) (1982).

Litchfield argues that section 106(2) was intended
to expand the definition of derivative works to
include any work based on a copyrighted work. She
argues that the similarities between her play and
the movie constitute the required "incorporation"
and show that E.T. was based on Lokey for purposes
of the statute. Litchfield apparently believes that
she does not have to show substantial similarity to
show that E.T. is a derivative work.

Litchfield cites no authority to support this novel
proposition. The little available authority suggests
that a work is not derivative unless it has been
substantially copied from the prior work. See Harry
Fox Agency, Inc. v. Mills Music, Inc., 543 F.Supp.
844, 849 (S.D.N.Y.1982), rev'd on other grounds, 720
F.2d 733 (2d Cir.1983), cert. granted sub nom. Mills
Music, Inc. v. Snyder, --- U.S. ----, 104 S.Ct.
1676, 80 L.Ed.2d 151 (1984); Reyher v. Children's
Television Workshop, 533 F.2d 87, 90 (2d Cir.),
cert. denied, 429 U.S. 980, 97 S.Ct. 492, 50 L.Ed.2d
588 (1976); 1 M. Nimmer, The Law of Copyright, Sec.
3.01 at 3-3 (1983 ed.).

Here's what the House Report the court cites says [4]:

To be an infringement the “derivative work” must be “based
upon the copyrighted work,” and the definition in section
101 refers to “a translation, musical arrangement, dramatization,
fictionalization, motion picture version, sound recording, art
reproduction, abridgment, condensation, or any other form in
which a work may be recast, transformed, or adapted.” Thus,
to constitute a violation of section 106 (2), the infringing
work must incorporate a portion of the copyrighted work in some
form; for example, a detailed commentary on a work or a
programmatic musical composition inspired by a novel would not
normally constitute infringements under this clause.

In Reyher v. Children's Television Workshop, 533 F.2d 87 [5], which is also cited by the Litchfield court, the court says:

Generally, "(a) work is not derivative unless it has substantially
copied from a prior work. If that which is borrowed consists merely
of ideas and not of the expression of ideas, then although the work
may have in part been derived from prior works, it is not a derivative
work." 1 M. Nimmer on Copyright § 39 at 166-167 (1975) (hereinafter "Nimmer")

The Perfect 10 case also comments on what is necessary in order for something to be a derivative work:

For example, a “Game Genie” device that allowed a player to alter
features of a Nintendo computer game did not infringe Nintendo’s right
to prepare derivative works because the Game Genie did not incorporate
any portion of the game itself. See Lewis Galoob Toys, Inc. v. Nintendo
of Am., Inc., 964 F.2d 965, 967 (9th Cir. 1992). We held that a copyright
holder’s right to create derivative works is not infringed unless the
alleged derivative work “incorporate[s] a protected work in some concrete
or permanent ‘form.’ ” Id.

> Dirty tests are often not enough in discussion around law

Right, and if I would not even attempt to offer a quick and dirty test to determine if a work that copies from another work is an infringing derivative work or if the copying can be excused by some factor such as fair use.

Sometimes, however, you can make simple quick rules that do work. In the case of derivative works, since for work X to be a derivative work of work Y, there must by copying from Y into X, it follows that if the creator of X works in such a way that they cannot have copied from Y, they cannot have created a derivative work.

I would have assumed most tech people already knew this, as it is the key idea that makes clean room design work. One team reverse engineers the specimen product, and writes a specification to cover whatever it is that the specimen does. Then a second team, that does not overlap the first, implements a product from that specification. The second team does not have access to any copyrighted material from the specimen, and so their implementation is not a derivative work, and they can market it safe from copyright liability. (This doesn't protect them from patent liability, though. There might also be contract issues if in order to get the specimen, the first team had to agree to not reverse engineer it. We're just talking about copyright here, though, and this procedure does deal with that risk).

[1] http://cdn.ca9.uscourts.gov/datastore/opinions/2007/12/03/06...

[2] http://en.wikipedia.org/wiki/Copyright_aspects_of_hyperlinki...

[3] http://openjurist.org/736/f2d/1352/litchfield-v-spielberg

[4] http://en.wikisource.org/wiki/Copyright_Law_Revision_(House_...

[5] http://openjurist.org/533/f2d/87