  Large millimeter interferometers are revealing a growing number of rotating
outflows, which are suggested to trace magneto-centrifugal disk winds (MHD
DWs). However, their impact on disk accretion is not yet well quantified. Here
we identify systematic biases in retrieving the true launch zone, magnetic
lever arm, and angular momentum flux of an MHD DW from apparent rotation
signatures. Synthetic position-velocity cuts are constructed from self-similar
MHD DWs over a broad range of parameters, and three different methods are
applied for estimating the specific angular momentum. We find that the launch
radius inferred using the well-known relation from Anderson et al. (2006) can
markedly differ from the true outermost launch radius $r_{out}$ of the DW. The
"double-peak separation" and "flow width" methods provide only a strict lower
limit to $r_{out}$. This bias is independent of angular resolution and can
reach a factor ten. In contrast, the "rotation curve" method gives a good
estimate of $r_{out}$ when the flow is well resolved, and an upper limit
otherwise. The magnetic lever arm is always underestimated. Only comparison
with synthetic predictions can take into account properly all observational
effects. As an application, we present a comparison with ALMA observations of
HH212 at resolutions from 250 au to 16 au, which represents the most stringent
observational test of MHD DW to date. This comparison confirms our predicted
biases for the double-peak separation method, and the large $r_{out}\sim40~$au
and small magnetic lever arm first suggested by Tabone et al. (2017). We also
derive the first accurate analytical expression for the fraction of disk
angular momentum extracted by an MHD disk wind of given radial extent, magnetic
lever arm, and mass flux. Application to HH212 confirms that MHD DWs are
serious candidates for the steady angular momentum extraction process in young
disks.
