Sesame seeds will have the best flavor if you buy untoasted sesame seeds and roast them yourself, because once they are toasted their flavor deteriorates. The Japanese markets that I have checked only had pre-toasted sesame seeds. Organic and naturel food stores have been a better source for me to find raw sesame seeds. If you live in San Francisco, then Rainbow Grocery is a good place to look for raw black sesame seeds (see Sources for Ingredients). However, even if you are not able to find untoasted sesame seeds, retoasting will help to perk up their flavor.
According to Hiroko Shimbo in “The Japanese Kitchen,” Japanese preparations always use unhulled sesame seeds. If it isn’t marked, you may be able to guess at the type by color: hulled sesame seeds are usually white and uniform in color; unhulled raw white sesame seeds will contain many different off-white shades, varying from white to off-white to beige to tan. Black sesame seeds always have the hull on, since the hull is what makes them black (the inside seed is white).
Heat a small pan on medium to medium high with no oil. Set the lid for the pan nearby. Add the sesame seeds when the pan is hot. Stir and shake constantly and watch carefully because sesame seeds burn easily. The sesame seeds may pop and jump out of the pan; this is a good thing — if they do cover the pan with the lid to avoid making a mess and shake every few seconds to redistribute them. Remove the lid when they stop popping and use a spatula to stir. If the sesame seeds don’t pop with in a minute or two, try a higher temperature. After the sesame seeds are cooked at low heat for several minutes, sometimes they won’t pop even after you turn up the heat. This is fine though the sesame seeds will take longer to toast.
Toast until they have a nutty aroma which smells similar to roasted peanuts or roasted peanut butter. The time depends on how how the heat is. Hotter temperatures will toast the seeds faster and require careful attention to stirring and shaking the pan in order to evenly brown the sesame seeds. On medium, it may take 3 minutes for a few Tbsp of sesame seeds. Larger quantities need longer, e.g. 10 minutes for 1 cup of sesame seeds. Toasting will give white sesame seeds a golden color. Black sesame seeds won’t change the color so be especially careful and make sure you use medium heat–you can tell when black sesame seeds are done by smell. When toasted, the sesame seeds will crush easily between your finger tips and smell nutty, toasted, and aromatic (not raw).
Remove the sesame seeds from the pan as soon as they are toasted because they will burn if you leave them in the hot pan.