Identifying seeds can be challenging. There are three basic factors that you have to keep in mind when identifying seeds. These are:
1. size
2. shape
3. texture
I could add color to this list, but color is too variable. It changes by the area where the seed was grown and the fact that color will change over time with age and storage.
You can use seed botanical "keys" to determine the species or through photos and drawings. A seed herbarium of positively identified species is one of the best means for comparison once you think you know the species.
Colorado State University offers courses and workshops in seed identification. Go to:
http://www.seedimages.com for more information on this subject.