Re: Hmmmm.
An ellipse tilted relative to our line of sight is still an ellipse (of different apparent shape). So there is no way to tell the tilt of a planetary orbital plane by purely geometric measurements.
However, if we get Doppler AND Astrometric data, (say for an intermediately tilted orbital plane) we get numbers in both the normal and transverse components, and with the aid of Newton's Laws, we can derive the true shape, independent of orientation, of the ellipse.
This technique has been worked out to high precision with binary stars. Also, we can always measure the orbital period, and in the special case of planets (which are always much, less massive than their primaries), the masses, also from Newton's Laws.