Yep, and it's in the interests of Google and users to keep things that way.
Google doesn't have to "detect." Google merely needs to "predict." If a mope like me can see text links for diet pills, mortgages, credit cards, etc. in the navigation bar of a weather site and deduce "Aha! Those are obviously paid links," it shouldn't be too hard for a team of search engineers to write an algorithm that makes the same educated guess. What's more, such blatantly off-topic links to e-commerce and affiliate sites don't have to result in penalties; they can simply be ignored.
No, every link that has rel="nofollow" is merely telling Google (and other search engines that recognize the nofollow attribute) that the Webmaster wants that link to be ignored. It certainly isn't likely to be a paid link unless the Webmaster is trying to cheat the buyer (and the buyer is dumb enough to let himself be cheated).
As for the idea (stated earlier in this thread) that outbound links without the nofollow attribute are risky, that's 180 degrees out of synch with common sense. Hypertext links (a.k.a. citations) are the most fundamental principle of the Web, they're the basis of Google's PageRank formula, and they're essential to a search engine's crawling and indexing of the Web.