I don't post SEO/search/webmaster videos here very often, because this is my personal G+ account, but this is an in-depth (8 minute!) video that explains some of the criteria we use when we decide whether a link is paid or not. Roughly, here are some of the important questions we ask:
- What is the value of the gift, product, or service?
- How close is the gift, product, or service to actual money?
- Is it an outright gift or a loan?
- Who is the intended audience?
- Is the intent of the gift to get links?
- Would the gift be a surprise to third party?
My guess is that anyone who has been involved with SEO for a while would intuitively understand these criteria, but it's helpful to clarify some of the questions we ask. Bear in mind that well over 99% of the time, paid links that pass PageRank are abundantly clear--because actual money is changing hands--plus we reserve the right to adapt our criteria when we see new types of spam or abuse.