 Solution

Coplanarity requires both $I=I_d$ and $\Omega ={\varphi }_d$, up to the orbital-plane orientation degeneracy. Under that hypothesis the measured planet angle must obeyFor a nearly edge-on belt this offset is small except close to the projected crossings where $\cos f=0$. Measurements at several orbital phases can therefore test the entire projected ellipse: a systematic offset from the predicted relation implies a nonzero mutual inclination. A single projected location generally cannot establish coplanarity because $f$, the front-back branch, and the nodal orientation can be degenerate.