I have been critical of her in the past, but Megan McArdle does a good job explaining how you assess tax incidence. This is good too (HT2MR):
Hillary Clinton's proposal is particularly stupid, in my humble opinion, because it tries to get the money back from the oil companies with a windfall profits tax. Tax incidence is tax incidence: if the oil companies can make consumers pay most of the excise tax, then probably consumers [sic?] can stick them with your windfall profits tax too. Meanwhile, the instinct to mess with the oil companies every time prices rise is thoroughly counterproductive. We (at least, those of us who want cheaper oil) want the oil companies out foraging for more supply. If you lower the returns on finding new oil, you kill their incentive to do so--more importantly, you kill the incentive of investors to give them capital to do so. All her plan does is make us take the trouble to build new administrative capacity to collect the tax, while keeping all the old administrative capacity for collecting the excise tax (since, after all, it's not actually going away permanently), while scaring the bejeesus out of investors. It's lose-lose-lose.