Just think it through.
If a contact's line is bending towards
your heading, you can assume its closing.
If the line is bending away from
your heading, you can assume its opening.
I use the word "assume" because the broadband trace only shows the relative bearing of the contact compared to your course and speed. That's why you need TMA. You figure out a rough course, speed, and range then tweak the solution over time to make it fit better.