When you do something like "test" in a where a is a list does python do a sequential search on the list or does it create a hash table representation to optimize the lookup? In the application I need this for I'll be doing a lot of lookups on the list so would it be best to do something like b = set(a) and then "test" in b? Also note that the list of values I'll have won't have duplicate data and I don't actually care about the order it's in; I just need to be able to check for the existence of a value.
"test" in a with a list a will do a linear search.  Setting up a hash table on the fly would be much more expensive than a linear search.  "test" in b on the other hand will do an amoirtised O(1) hash look-up.
In the case you describe, there doesn't seem to be a reason to use a list over a set.
Fastest way to search a list in python