IO540, I think you are referring to this sort of situation:
where HDG/TAS and TR/GS have got the same length (or the triangle is isosceles, but I'm getting pedantic now) so that ground speed and airspeed are the same. In this case, the wind is perpendicular to the bisector - or equivalently, to the vector sum of heading and track.
Anyway, this is all to say that the only way to never get it wrong is to have always well clear in mind the vectors' triangle.
Cheers!
Deeday