  The angular power spectrum of the cosmic microwave background temperature
anisotropy observed by WMAP has an anomalous dip at l~20 and bump at l~40. One
explanation for this structure is the presence of features in the primordial
curvature power spectrum, possibly caused by a step in the inflationary
potential. The detection of these features is only marginally significant from
temperature data alone. However, the inflationary feature hypothesis predicts a
specific shape for the E-mode polarization power spectrum with a structure
similar to that observed in temperature at l~20-40. Measurement of the CMB
polarization on few-degree scales can therefore be used as a consistency check
of the hypothesis. The Planck satellite has the statistical sensitivity to
confirm or rule out the model that best fits the temperature features with 3
sigma significance, assuming all other parameters are known. With a cosmic
variance limited experiment, this significance improves to 8 sigma. For tests
of inflationary models that can explain both the dip and bump in temperature,
the primary source of uncertainty is confusion with polarization features
created by a complex reionization history, which at most reduces the
significance to 2.5 sigma for Planck and 5-6 sigma for an ideal experiment.
Smoothing of the polarization spectrum by a large tensor component only
slightly reduces the ability of polarization to test for inflationary features,
as does requiring that polarization is consistent with the observed temperature
spectrum given the expected low level of TE correlation on few-degree scales. A
future polarization satellite would enable a decisive test of the feature
hypothesis and provide complementary information about the shape of a possible
step in the inflationary potential. (Abridged.)
