There are two ways I know of to tell direction of flow:
1) Observation of movement in the jet over time.
2) Spectral line shift for jets that are not tangent to our line of sight.
(I've heard that movement has actually been observed in some jets, but I guess I wouldn't bet my life on it.)
If your prediction is correct, then half of all jets should be inflowing. I've never heard of an observation that mentioned anything but outflow when talking about a jet.
LB
