How to look up seal script characters?
Looking up seal characters is not as hopeless as other answers seem to indicate, even though it is considerably harder than looking up modern characters. In any case, the lookup needs to consist of identifying which component of the seal character is the radical and finding the character in the corresponding section. Identification of the radical needs to be done by trial and error, as there are no fixed rules that an be applied when you don't know the character in the first place.

Let us start with the slowest, traditional method and see how it can be made faster using electronic dictionaries:


Use a paper edition of 說文解字 Shuōwén Jiězì that contains a seal radical index. Note that most modern editions of Shuōwén have seal script headwords, but their indices use modern characters.
Use an online list of seal radicals to find out the number of the radical, and find the character in the corresponding section of Shuōwén Jiězì. The radical numbers are equivalent to Shuōwén section numbers (this is where the name 部首 “section headers” comes from).
Use Wenlin and choose List -> D. Shuowen Seal Radicals. This will give you a list of 540 seal radicals, and clicking on any of them will give you all the seal characters listed under this radical. This is much faster than looking up the character in Shuōwén, especially when you are not sure which component is the radical (部首). Note that Wenlin lists are based on a Qīng Dynasty commentary on Shuōwén, 說文解字·注, not the original, Hàn-era 說文解字.
The problem with the original Shuōwén radical list is that it is arranged semantically, in a way that is not helpful for somebody who doesn't know the radicals by heart. Therefore, I made two other indices that can be opened in Wenlin and used in the same way as above:


the list of Shuōwén seal radicals ordered by approximate stroke count, and
the list of Shuōwén seal radicals ordered by their token frequency in Classical Chinese texts (based on Jun Da's Classical Chinese Character Frequency List).



The former list begins with radicals that are the simplest graphically, and the latter one presents the most frequent radicals first, which makes it much easier to find the radical you're looking for.