In Doing Keyword Research...

Think about what is going through your readers heads' when they type in the keyword. Are they thinking about a specific product, or are they just looking for information?

There are certain indicators that clearly indicate you have a buying keyword. As a previous poster indicated, the number of PPC ads that show in the search results is a great clue to showing whether this is a buying keyword. If someone searches for the "Best (whatever)", this is someone who is likely going to make a purchase soon.

People searching for a specific product in a niche are definitely in the buying stage of the process.

The word "free" usually indicates that someone is not willing to spend money on a product. This can work for CPA email submits and other smaller CPA offers, but it won't work for anything where the customer has to spend money.

I still think the best way to determine if a keyword is a buying keyword is to place yourself in the buyer's shoes. When they search for that keyword, are they still searching for information, or are they about to make a purchase? That is the best way to know whether you have a buying keyword or information keyword.