That is a tough question.
A link farm consists of some, but not all of the following criteria:
- Accepts listings automatically
- Links to pharmacy, adult, casino and other bad neighborhoods
- Consists of, or has entire pages that consist of only links
- Links are usually not categorized. If they are, they are incorrectly categorized
Don't confuse a link farm with a properly categorized, properly edited and reviewed web directory.
I hope this helps.