In order for a field in the relationship graph to appear with a "single line connector" instead of "crows feet", it must either be defined as an auto-entered serial number field or have a unique values validation specified.
Filemaker doesn't 'know' if a field is a parent key. It uses a basic algorithm along the lines of (I'm quoting the first post I found on the subject (David Jondreauat on fmforums):
Straight line: When the field is defined as Auto-Enter or Validate Unique.
T or Pipe: When field is defined as global or unstored.
Crow's feet: Everything else
So- if you're seeing the crow's foot (or triangle), I'm guessing you have not set it to auto-enter serial numbers or otherwise be validate as unique in the 'options.'
Here's another post where Phil also says:
Just because FileMaker shows "crows feet" on both ends of the relationship line, this does not mean that you have a many to many relationship nor that you have the wrong relationship set up. You get the "one to many look" of a single connecter line only if that field is an auto-entered serial number or if it has a unique values validation specified for it. That's info that "tells" FileMaker that it has to be one to many.
Edit: Aye Phil got here first with a pithier response to boot.