Wallcraft makes a very good suggestion.
There is no easy way of searching for all the samples on the Kindle directly. If you don't have many pages of kindle books then you can easily see samples on your kindle as they all have the word "sample" preceding them. This is no use to me as I have 60 pages of books to look through.
So I store all the samples into a collection on the kindle. If I buy a sample book then I remember to delete the sample off the kindle. Bit of a faff but it is a system that works for me. Be nice if the kindle automatically removed samples when you buy the same book.