They seem to be similar, but perhaps there is some distinction. When you're infield is it possible to tell the difference (if there is any)? Or is this just over-analysis?

A shit test and an IOD can be a woman saying exacly the same thing, but what she is looking for in your response is what determines what it is.

For example.

HB: I only date men with college degrees.

If she REALLY only does that, and she knows you dont have one, its an IOD.
It might be a ST though, if she doesnt actually mean it.

Usually IODs are passive disqualifiers chicks throw at you to kindly let you know that they are not intersted.

Examples, youre too old for me, I prefer muscular guys, or Im not looking for a bf.

A shit test I consider an IOI because a woman wont test a guy she has absolute 0 interest in.

Examples, are you always this hung on yourself? Do you always drink this much? I need to have sex everyday at least twice.

They key is not her exact words, but how she is saying them. If she says them in a non serious/borderline joking way, shes flirting. Not insulting your awesomeness or drunkeness. You should be getting more STs then IODs. If not, you might be creeping HBs out. If you recognize a ST, be happy, but dont fail it either.

Thanks Slipknot, that's a good explanation.