Mike Johnduff, a Princeton grad student in English whose "Working Notes" blog includes a number of interesting short essays on figures in Continental philosophy (e.g. Heidegger, Marx, Foucault, etc.), has written several pieces on Freud, including this article what we can know of psychological drives according to Freud.
Though drives are determined by that somatic process whose being stimulated is represented by the instinct (their source), in mental life itself we know them only by their attempt (within mental life) to remove that stimulation of the somatic process--what Freud calls the "aim" of the drive. (This is the reason why later Freud will say that drives are also only present to the unconscious--just as much as consciousness--through their "representatives.")
To put it a different way, though drives get their character (roughly, "what distinguishes one from another," as Freud goes on to say) from the sort of stimulation which they represent, they can't be seen in mental life except in terms of how they try to rid themselves of this character. We can't look at a drive then and see how the character was formed. We can only postulate that it was formed from looking at how it proceeds to try and get rid of this form--that is, how it uses this form.
What this means is that we don't see drives like the sex and death drives. We only see what someone seems to be aiming at, which is the removal of some tension or another that's been built up as a result of the alleged drive. Moreover, we can't just characterize the tensions that people are trying to address and call those the drives, because these tensions are just the results, the "representatives" of more basic drives, which can presumably manifest themselves in different ways.
To me, this seems again like Freud has created a structure that is internally complex enough that there's not going to be an objective way to figure out what drives there are, so we won't be able to confirm or falsify his theory about these empirically.
Johnduff's article goes a bit into issues of translation and is interested in explaining what Freud's view is rather than critiquing it; this seems typical (based on my brief scan) of most of his blog posts: they are "working notes" for his coursework, and to my mind don't go quite far enough in laying out the views in question in a web-friendly manner. If a commentary is not much much clearer and easier to scan then the primary source, then why bother with the secondary source at all? However, he does bring some interesting issues to his readers' attention, and given the obscurity of his interests (Derridas doesn't go out of his way to be analytic-philosopher-friendly), we should be grateful for what effort Johnduff does put in to bring ignorant readers up to speed. I'll definitely look back here when we get to the more obscure Continental figures in our podcast.